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New York has paused to remember the collapse of the World Trade Center on 11 September 2001 - an attack that killed nearly 3,000 people.

Four moments of silence have been held to mark the times when four hijacked passenger planes hit the Twin Towers, the Pentagon and a Pennsylvania field.

President Bush dedicated a new memorial at the Pentagon, where 184 died.

"The worst day in America's history saw some of the bravest acts in America's history," he said.

Presidential candidates Barack Obama and John McCain are attending a ceremony at Ground Zero in New York.

New York City Mayor Michael Bloomberg opened Thursday's memorial event at Ground Zero, where families of the victims read out a roll call of those who died.

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