Michael Bloomberg, New York’s mayor, led the first of four silences at "Ground Zero" which marked the moments the Twin Towers were hit by the hijacked airliners and the moments they collapsed.

Family members and students representing more than 90 countries, including Britain, that lost citizens in the attack read the names of the 2,974 people who died – a tribute which was due to last about four hours.

At the New York Stock Exchange, the usually frenetic activity came to an abrupt halt as traders bowed their heads in silence.

In Washington, President Bush led for the last time a White House gathering to remember an event that defined his presidency. Services were also held in Pennsylvania and at the Pentagon, where a new memorial was unveiled.

Later in the day, the presidential candidates John McCain abdBarack Obama were due at Ground Zero to pay silent respects together "as Americans".