Relatives of those who died in the September 11 terror attacks gathered yesterday to remember their loved ones on the seventh anniversary of a day that "changed the world forever".

New York City mayor Michael Bloomberg led the first of four silences at Ground Zero, which marked the moments the Twin Towers of the World Trade Centre were hit by two hijacked airliners and the moments they collapsed.

He said: "Today marks the seventh anniversary of the day our world was broken a day that began like any other and ended as none ever has."

Family members and students representing more than 90 countries, including Britain, which lost 67 citizens in the attacks, read the names of 2974 people who died in the attacks on the US. The tribute lasted about four hours.

Other solemn remembrance services were held where two other hijacked airliners crashed in Pennsylvania and in Washington DC.

The attacks, which triggered the US-led invasion of Afghanistan and Iraq and President George W Bush's "war on terror", are regarded as the defining moment of his time in office.

US presidential rivals John McCain and Barack Obama called a temporary halt to their fierce political skirmishing yesterday in honour of those who died on September 11.

Mr McCain was among several hundred people who attended a ceremony in Shanksville, a town in south-western Pennsylvania where the fourth hijacked plane crashed into a field, killing 40 passengers and crew.

He later flew to New York to join Mr Obama for a visit to Ground Zero, where they both laid wreaths.

In a park near Ground Zero, relatives of the dead held up portraits of their loved ones as bagpipes and drums sounded from police and fire department bands.

A ramp with flags of the world led into the pit beneath the site where a circular reflecting pool contained two squares representing the footprints of each of the twin towers.

Among the speakers at Ground Zero was Alex Salamone, who was a young boy when his father went to work at the World Trade Centre seven years ago and never came home.

"I remember playing in the yard with him. I remember him pulling my wagon. He was strong. He always made me feel safe," said Mr Salamone, wearing the soccer jersey of his father, John. "I wish I could remember more, but we were so young when he died."

Edward Bracken, who lost his sister, Lucy A Fishman, said she was "murdered by coward men using their religion to say they are right and we are wrong", then added: "Pray for the men and women who sleep on the ground every night in the Middle East to keep our world safe."

In Washington, Mr Bush unveiled a new memorial to 184 people who died at the Department of Defence at the Pentagon.

Dedicating the memorial, he said: "Seven years ago at this hour a doomed airliner plunged from the sky, split the rock and steel of this building and changed our world forever.

"The years that followed have seen justice delivered to evil men in battles fought in distant lands."

He went on: "The worst day in America's history saw some of the bravest acts.

"Since 9/11 our troops have taken the fight to the terrorists abroad so we do not have to face them at home.

"Thanks to the brave men and women, and all those who work to keep us safe there, has not been another attack on our soil in 2557 days."

The memorial, by New York designers Julie Beckman and Keith Kaseman, features maple trees and light pools in a gravel park. Benches are arranged according to the ages of the victims.

The attack on the Pentagon took place at 9.37am, when American Airlines Flight 77 from Washington's Dulles International Airport smashed into the walls of the US military headquarters in Washington DC.

The crash killed 125 people in the Pentagon, along with the plane's 59 passengers and crew and the five hijackers.

Memorials are years away from being built in New York and in Pennsylvania, where Flight 93 came down in a field after passengers reportedly stormed the cockpit to thwart terrorists' plans to use that plane as a weapon like the others.

As in previous years, two bright blue beams of light were to be shone at night on the New York City skyline, in memory of the fallen Twin Towers.

In Britain, Robert Tuttle, the US ambassador to Britain, laid a wreath at a memorial near the US Embassy in Grosvenor Square, central London, at 8am to honour the British victims.

Prime Minister Gordon Brown pledged the continuing "support of the British people for America". He said he would be phoning Mr Bush later yesterday and urged people to remember all those who died in the attacks.