11 September 2011
Last updated at 18:02
The US is marking the 10th anniversary of the 9/11 attacks on New York, Washington and Pennsylvania in which nearly 3,000 people perished.
President Barack Obama and former President George W Bush attended the first ceremony, at the site of the World Trade Center in New York.
Mourners filed into a new memorial at Ground Zero, on which the names of the dead are engraved.
President Obama read from the Bible, a passage from Psalm 46, which speaks of God's refuge and strength, "a very present help in trouble".
Accompanied by his wife Michelle, he then travelled on to Pennsylvania, where the couple laid a wreath of white flowers at the site where the fourth plane was forced down in a Shanksville field by passengers who fought back against the hijackers.
Security was tight following warnings of a possible al-Qaeda attack.
The CIA received a warning last week that al-Qaeda may have sent attackers, some of them possibly US citizens, to bomb one of the cities.
As on previous anniversaries, the names of all the victims were read out at the site of the World Trade Center.
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 memorial pools at Ground Zero.
Relatives of victims also gathered in Washington and Pennsylvania, where hijacked planes also crashed on 9/11.
In a speech at the Pentagon, Vice-President Joe Biden commended America's armed forces for their service and their sacrifices since the attacks.
In the early hours of Sunday the Tribute in Lights was illuminated next to the Statue of Liberty and the One World Trade Center.