America led the tributes of those lost, with families of the dead joining President Barack Obama and former President George Bush at a ceremony at the site of the World Trade Centre in New York, where 2,753 were killed in the 2001 atrocity.
Relatives o some of the 67 Britons were in New York for the solemn occasion. Others attended commemoration events in the UK.
On both sides of the Atlantic moments of remembrance were observed to mark the 10th anniversary of the attack.
At 8.46am in New York people united in silent contemplation.
This was the time at which the first hijacked plane slammed into the north tower of the World Trade Centre.
What followed amounted to the worst terrorist atrocity carried out on the US mainland.
Al Qaida terrorists flew passenger jets into both towers at the World Trade Centre in New York and the Pentagon in the nation’s capital.
A fourth plane crashed into a field in Pennsylvania after passengers stormed the cockpit in a desperate effort to prevent hijackers hitting a fourth target, assumed to be Washington’s US Capitol.
Britain suffered more losses in the September 11 attacks than any other country apart from America.
Around 30 of the bereaved British families were due to attend the official remembrance ceremony in the UK.
The relatives of a further 10 UK victims are believed to have travelled to New York for today’s event at Ground Zero, the former site of the World Trade Centre.
During the ceremony in New York, the names of those who died were read out while bells tolled across the city.
Some 2,753 died in New York, 184 in Washington and 40 in Pennsylvania. Among the dead were 343 firefighters, 37 police officers, two FBI officers and one US secret service agent.
Ten years on, the effects of the attacks are still deeply felt in America.
And the fallout from the atrocity continues to reverberate around the world, with US troops engaged in operations in Afghanistan and Iraq – conflicts launched in the aftermath of September 11.
During a weekly radio address yesterday, Mr Obama – a little known state senator at the time of the attacks – called for national unity.
“A decade after 9/11, it’s clear for all the world to see – the terrorists who attacked us that September morning are no match for the character of our people, the resilience of our nation, or the endurance of our values,” he said.
Mr Obama’s two immediate predecessors, presidents George Bush and Bill Clinton, travelled to Pennsylvania yesterday to pay tribute to the “heroes” of Flight 93 – the domestic jet that was brought down by passengers before it could reach its target.
Security in both Washington and New York has been high throughout the weekend amid concern that terrorists may try to use the occasion to mount a fresh assault.
These fears were heightened by uncorroborated reports from a CIA informant that al Qaida bosses may have dispatched operatives into the US to set off a lorry bomb.
In Lower Manhattan, pavements were packed with people who turned out to pay their respects.
Crowds were slow moving as people attempted to get as close as possible to Ground Zero.
At St Paul’s Chapel opposite the site of the World Trade Centre, hundreds of white ribbons had been tied to railings in memory of lost loved ones.
Some people were in tears, other stopped for photographs.
Heavily armed police mixed with patrolmen at metal barriers – a sign of on-going security concerns.
Cheers went up from the crowd as the national anthem was sung by a military choir.
As the bells from St Paul’s Chapel rang out, people in the crowd bowed their head and observed the silence impeccably.
Many wept as the long list of names of the dead began to be read out by bereaved family members.
Tsion Evans, 51, was among those in the crowd listening.
She attended with her daughter Nia, 25, to honour the memory of friends who died in the attack.
Nia, who was in high school in 2001, explained: “We are here to show there is still a lot of spirit and love. And also to support the police and fire departments – we give our thanks to them.
“We will never forget those who died.”
At Ground Zero, relatives of those who died walked around the memorial that has been created at the site to see it for the first time.
The names of the victims are carved into low walls surrounding two voids where the towers once stood, and the words will be lit up at night.
Water cascades down the walls of each of the voids into a pool.
Some of the relatives took rubbings of their loved ones’ names, while others simply placed their hands on the carved words.