-America remembers the day 10 years ago when terrorists flew planes into the World Trade Center and Pentagon
-Bush and Clinton among guests at dedication in Shanksville, Pennsylvania, of memorial to Flight 93 hijackers
-Other memorials planned for New York and Washington D.C. as America mourns almost 3,000 victims
By Paul Bentley, Rachel Quigley And Mark Duell
Mourning: Robert Peraza, who lost his son Robert David Peraza, pauses at his son's name at the North Pool of the 9/11 Memorial today
Families of the thousands of victims killed in New York on 9/11 gathered this morning at Ground Zero as America began a sombre day of tributes to those who lost their lives in the terror attacks which shocked the world ten years ago today.
Thousands of grieving mothers, fathers and children filled the memorial site to lead the U.S. in pausing and reflecting on the decade since terrorists caused the Twin Towers to crumble, flew into the side of the Pentagon and crashed a plane in Pennsylvania.
At the ceremony today, moments of silence will be held to mourn those who perished as each of the planes crashed and the two towers went down, while President Barack Obama and his predecessor George W. Bush will deliver readings and the names of the 2,753 who died will be read aloud.
While New York will form the focus of the memorial day, respects will be paid throughout the country, with events at the Pentagon and in Pennsylvania also poignantly marking the passing of innocent Americans a decade ago.
Dressed in black: President Obama and Michelle arrive at Andrews Air Force Base this morning before boarding Air Force One for their flight to New York
Patriotic: An American flag is unfurled at the Pentagon in Arlington, Virginia, to commemorate those who lost their lives in the attacks
Day of remembrance: Dawn breaks at Ground Zero today, ten years to the day after the 9/11 terror attacks that shook America
Into the sky: The 'Tribute in Light' shines above lower Manhattan, the Statue of Liberty, and One World Trade Center, left, on Saturday in New York
Lights of remembrance: The Tribute in Light as seen from Liberty State Park, left, and a view of it over the Brooklyn Bridge on the right
Light fantastic: The Tribute in Light uses 88 powerful beams and has been running every year to mark the anniversary of the attacks
On the ground: National Guard troops stand at the World Trade Center as the tribute lights are turned on to remember the victims of the 9/11 attacks on Saturday night
All ready: Water flows in the fountains of the National September 11 Memorial in New York on Saturday, ahead of the 10th anniversary of 9/11
As authorities stepped up security throughout New York City, the National September 11 Memorial and Museum memorial service will see families gather throughout the day.
A moment of silence will be held at 8.46am, when the first plane crashed into the North Tower, and then the names of the victims will be read.
Further moments of silence will be held to mark the other attacks in New York, Washington DC, and Pennsylvania at 9.03am, 9.36am, 9.59am, 10.03am and 10.28am.
The annual 'Tribute in Light' will then begin from the WTC site at sundown, visible for more than 60 miles. Two blue beams, made up of 7,000 watt bulbs, were switched on for the first time this year on Tuesday night.
As the victims' families gather to honour their lost ones, law enforcement agencies around the country have stepped up security at airports, nuclear plants, train stations and elsewhere in anticipation of possible anniversary attacks.
President Obama met with his national security team on Saturday, but the White House released no new information about possible threats.
A statement insisted counterterrorism efforts were working well and would not ease in the weeks and months ahead.
Residents and workers in the area will be required to carry identification to gain access with 20 downtown streets planned for closure.
Today marks the opening of the memorial and museum, set in the footprints of the original twin towers among a small forest of oak trees in an eight-acre plaza.
The memorial, which opens to the public tomorrow, features two 50ft-deep pools, each containing fountains, along with a museum with exhibitions and artefacts to teach visitors about the events of September 11. The pools have the victims' names etched around its perimeter.
Various American leaders have taken time this week to speak about the attacks and the importance of remembering what happened ten years ago.
Remembered: Former U.S. presidents George W. Bush, left, and Bill Clinton, right, and former first lady Laura Bush, centre, bow their heads during ceremonies marking the 10th anniversary of the 9/11 attack in Shanksville, Pennsylvania, on Saturday
Crying: Former First lady Laura Bush, left, and Jill Biden, right, wipe tears away at the ceremony in Shanksville
Tears: Former U.S. president Bill Clinton wipes away a tear during ceremonies for the opening of the Flight 93 National Memorial and embraces Mr Bush
Vice President Joe Biden holds his head in his hand during the dedication ceremony of the permanent Flight 93 National Memorial
Vigil: Visitors move amongst candle-lit luminarias at the Wall of Names at phase 1 of the Flight 93 National Memorial near the crash site of Flight 93 in Shanksville, Pennsylvania
President Obama has sought to strike a balance between remembering and moving forward, while also trying to summon the feeling of unity that existed during the dark days after terrorists killed nearly 3,000 Americans.
'They wanted to deprive us of the unity that defines us as a people. But we will not succumb to division or suspicion. We are Americans, and we are stronger and safer when we stay true to the values, freedoms and diversity that make us unique among nations,' he said.
Mr Obama also thanked American troops who have served in two long wars in Iraq and Afghanistan and added: 'We're doing everything in our power to protect our people.
No matter what comes our way, as a resilient nation, we will carry on.'
Former New York City Mayor Rudy Giuliani, who led New York in the days after the attacks, voiced some of the same themes in the Republican Party's weekly radio and Internet address. He said that on 9/11 terrorists had achieved their goal of killing Americans - but failed to destroy the American spirit.
'The country was not broken, but rather, it was more united in the days after September 11 than at any time in my lifetime. We displayed heroic spirit in many ways, but perhaps the most heroic was the unity of spirit that we shared as Americans,' he said.
'The American people demonstrated one of the most basic values that we share - our love of freedom and the value we place on individual human life.'
Meanwhile, former President George W. Bush paid tribute to the victims of Flight 93 yesterday, describing their actions as some of the most courageous in U.S. history.
Mr Bush was joined by former president Bill Clinton to lead a silent tribute to the victims of September 11 at the Pentagon and in Pennsylvania the day before the official anniversary of the terror attacks.
Mr Bush said the defiance of passengers aboard the doomed plane was a shining example of democracy in action.
Serene: Views of the World Trade Center South Tower memorial pool at the National September 11 Memorial and Museum. The names of the victims are etched into bronze panels that surround them
Protection: A Port Authority officer provides security at the South Pool at the World Trade Center construction site in New York. It will be unveiled to the victims' families on Sunday September 11
More than 4,000 people, including relatives of those killed when the plane crashed into a rural Pennsylvania field, attended the service.
Mr Bush, who was joined by his wife Laura, placed a wreath of white flowers by the 9/11 memorial stone embedded in the wall outside Corridor 4, which is close to where the hijacked American Airlines Flight 77 crashed into the building, killing 184 people.
Also at Saturday's brief ceremony were Defence Secretary Leon Panetta, former Pentagon chief Donald H. Rumsfeld and Joint Chiefs of Staff Chairman Admiral Mike Mullen.
A long white stone wall bearing the names of those who struggled with al-Qaeda terrorists on the fourth airliner to be hijacked on September 11, 2001, was unveiled on the rural Pennsylvania field where the Boeing 757 crashed.
Current vice president Joe Biden joined the former presidents, families of the victims and several hundred others -- many in patriotic T-shirts or holding US flags under a slate grey sky.
During the ceremony, the names of the 40 victims were read out, one by one, accompanied by chimes.
Today President Obama will also join a two-hour commemorative service at the spot where Flight 93 went down.
The Flight 93 National Memorial currently includes an elongated walkway which sweeps past a circular field marked by a wreath-bedecked 17-ton boulder.
The adjoining wall bearing the names of the dead retraces the direction in which Flight 93 came down. Planted by the entry to the walkway are three young elm trees, representing the three 9/11 sites.
Notable upon the stage yesterday were the flags of Germany, Japan and New Zealand - in remembrance of wine merchant Christian Adams, 37, student Toshiya Kuge, 20, and lawyer Alan Anthony Beaven, 48, the non-native-born Americans on the flight.
Thoughtful: President Barack Obama and First Lady Michelle Obama visit section 60 at Arlington National Cemetery on September 10, which contains service members killed in the Iraq and Afghanistan wars
Comforting: President Obama hugs a visitor as First Lady Michelle Obama watches during the visit to Arlington National Cemetery in Washington
Service: Flags are carried into St. Patrick's Cathedral during a memorial ceremony on Saturday to honor New York firefighters that were killed in the attacks on the World Trade Center
Tribute from afar: U.S. soldiers from Task Force Bronco pause for a prayer in silence during a memorial ceremony to commemorate the 9/11 attacks, at a U.S. military camp FOB Shinwar in Afghanistan
A U.S. Navy brass quintet in crisp white uniforms played a prelude. US park rangers and FBI agents raised the national flag. Award-winning bagpiper Bruce Liberati performed, as did Canadian singer-songwriter Sarah McLachlan.
In the aftermath of 9/11, local volunteers took on the task of greeting visitors and maintaining a makeshift memorial along the chain-link fence that overlooks what some call 'America's first battlefield against terrorism'.
On Friday, family members of those who died on Flight 93 visited the site, read the guestbook and viewed the many mementos left by people from all over the world who have come to pay their respects.
Relatives shed some tears, but they also celebrated the spirit of the guestbook - a rare feeling that people from vastly different walks of life had come together.
'I don't focus on what happened. You can't change that,' said Lorne Lyles, whose wife, CeeCee Ross Lyles, had been working as a United Airlines flight attendant for only nine months on that September morning in 2001.
'Coming here is more of a celebratory thing. She's been memorialised,' Mr Lyles said. 'Just to see the outpouring from all over the world is touching. You really do have some caring people in the world.'
Secretary of the Interior Ken Salazar spoke at the site on Friday. He noted that for all the progress on the memorial, there's still work to be done. When it is finished, it will include a Tower of Voices with 40 wind chimes.
Public and private donors have contributed $52 million, but $10 million more is needed to build a true visitors centre and to finish landscaping, Mr Salazar said. 'We will not be able to complete the site' without additional funding', he said.
People walk through the Empty Sky memorial at Liberty State Park, in Jersey City, N.J., Saturday, Sept. 10, 2011, during the dedication of the memorial. Thousands gathered at a New Jersey park
New Jersey Governor Chris Christie, lays a white rose on wreckage pulled from Ground Zero during the memorial dedication to the Empty Sky Memorial at Liberty State Park
Linking arms: To commemorate the 10 year anniversary of the September 11 attacks, people participate in the 'Hand-In-Hand 9/11' ceremony by holding hands for a moment of silence at 8:46am, the time that the north tower of the World Trade Center was hit
Meanwhile, 2,753 Flags of Honour - each baring the names of 9/11 victims in patriotic stripes of red and blue - are standing at the tip of Manhattan as New York City marks the tenth anniversary of the terrorist attacks.
The NYC Memorial Field, part of a five-day installation, was erected to give New Yorkers a public place to gather in remembrance of those who were killed in the horrific acts of September 11, 2001.
On Friday in midtown Manhattan, 2,753 empty chairs, representing the lives lost on 9/11, were set to face south toward the World Trade on Bryant Park's lawn for part of a project called Ten Years Later, A Tribute 9/11.
Meanwhile, actors and performers from the Broadway community gathered at Times Square in costume for Broadway Unites: 9/11 Day of Service and Remembrance ceremony.
Yesterday morning at precisely 8:46am, thousands gathered to grasp hands and form a human chain to commemorate 9/11 at the tip of the Lower Manhattan waterfront heading north.
Organisers at Manhattan Community Board 1 said the event is open for those who feel excluded from today's official 9/11 Memorial ceremony, which is only open to families of the victims. Events to mark the tenth anniversary will go on throughout today in Manhattan.
The Metropolitan Museum of Art will display the 9/11 Peace Story Quilt with an accompanying programme throughout the afternoon.
Graduate students from New York University will read poetry from the quilt and a free concert will be performed. Created in collaboration with New York City students aged between 8 and 19, the quilt was made to convey the importance of communication among cultures and religions to achieve peace.
Comrades: James Wathen, right, from the Houston Fire Department in Texas attends a memorial service outside Saint Patrick's Cathedral in New York
343: Firefighters carry a banner with the number of firefighters killed on 9/11, during a memorial ceremony to honor New York firefighters that were killed in the attacks on the World Trade Center at St Patrick's Cathedral
The 92nd Street Y will have free memorial services on 9/11 for families at 2pm and for other adults at 3 pm. A talk will also be given by photographer Joel Meyerowitz, creator of the World Trade Center archive, at 7:30pm.
The New-York Historical Society will showcase a selection of photos taken during the immediate aftermath of the attacks on the World Trade Center. The Remembering 9/11 photo exhibition will be on view until April 12.
A film titled World Trade Center: All Times, based on a 10-year project by Fred J. DeVito that began as a way to remember the events and how they shaped the lives of Americans, will play at the Big Screen Plaza in Manhattan's Flatiron district.
The New York Mets will hold a tribute at Citi Field at 7:30pm, half an hour before their game against the Chicago Cubs begins. John Franco will throw the first pitch to Mike Piazza - both members of the 2001 team.
An Evening of Light 10th Anniversary Gala will be also held at Capitale at 8pm.
The fundraiser event is for Tuesday’s Children, a non-profit family service organisation which helps those affected by the attacks on 9/11.
FDNY 10th anniversary memorial service honouring members lost at WTC, a free ceremony at St Patrick’s Cathedral, will be held from from 2-4pm, honouring the 343 FDNY families that lost a loved one at the World Trade Center. The ceremony will be shown on large TV monitors in midtown Manhattan.
Later in the day, the famous church will hold a free concert given by the Young Peoples Chorus of New York, the New York Choral Society, and Cathedral Choir of St Patrick.