A weekend of ceremonies has begun in the United States to mark the 10th anniversary of the September 11 attacks on New York, Washington and Pennsylvania in 2001.
Security is tight following warnings of a possible al-Qaeda attack.
An official memorial to those who died is to be unveiled at the site of the World Trade Center, whose twin towers were destroyed in the attacks.
New York's National September 11 Memorial, features two reflecting pools, each almost an acre in size, in the footprints of twin towers reports the BBC.
The names of those who died on 9/11, as well as the six people killed in the 1993 World Trade Center bombing, are inscribed on the edge of the pools.
As in previous anniversaries, the names of all the victims will be read out at the New York event.
Also on Saturday, firefighters from around the world attended a memorial service in New York's St Patrick's Cathedral to honour the 343 firefighters who died on 9/11 while rescuing people from the World Trade Center towers.
On Sunday, President Barack Obama will travel to all three sites in New York, Washington and Pennsylvania where 2976 people died.
He will be joined in New York by former President George Bush.
Security has been tightened in New York and Washington after the reported threat in the run-up to the anniversary. US officials believe al-Qaeda may have sent attackers to bomb one of the cities.
''There should be no doubt: today, America is stronger and al-Qaeda is on the path to defeat, President Obama said in his weekly address on Saturday.
''Yes we face a determined foe, and make no mistake - they will keep trying to hit us again. But as we are showing again this weekend, we remain vigilant,'' he said.
''As a resilient nation, we will carry on,'' he said.