Fifty-two people died when four British Muslim suicide bombers, with explosives packed into rucksacks, blew themselves up inside three packed London underground trains and a double-decker bus on July 7, 2005.
It was the worst-ever terrorist attack on British soil, as well as Europe's first experience with a suicide bombing.
Midway through a day full of remembrance events, the nation observed two minutes' silence at noon, signalled by the 12 midday chimes from Big Ben, in the heart of the capital.
Prime Minister Tony Blair, who joined firefighters for the commemoration, said Friday was "a chance for the whole nation to come together to offer comfort and support to those who lost loved ones or were injured".
"This is a time when our country unites across all races, religions and divides and stands in solidarity with all those who have suffered so much in sympathy with them and in defence of the values we share."
Solemn commemorations began during the morning rush hour at King's Cross station, where a year earlier the four bombers - Mohammad Sidique Khan, Shehzad Tanweer, Hasib Hussain and Jermaine Lindsay - stepped off a commuter train and split up to carry out their lethal mission.
On the airwaves, Metropolitan Police Commissioner Sir Ian Blair warned that London - home to seven million people from countless nationalities, faiths and ethnic groups - was not immune from another attack.
"Since July 7, the threat has palpably increased," Sir Blair told BBC radio. "I fear we have to accept that we live in an age ... when the threat of an attack getting through is very real."
"The threat is very grim, there's no doubt about it. There are, as we speak, people in the United Kingdom planning further atrocities."
Extra police were seen deployed at underground stations, but Transport for London, which oversees the capital's mass transit system, said there was no sign of commuters and tourists shunning the Tube and buses.
"It's been very much business as usual," a spokesman said.
Bomber's videotape
On the eve of the commemorations, Tanweer made a virtual return from the dead by way of a chilling videotape, aired on British television.
"What you have witnessed now is only the beginning of a string of attacks that will continue and become stronger," said Tanweer on the tape, which also featured Al-Qaeda's number-two Ayman al-Zawahiri and was apparently put together within the past five weeks.
On Friday, more details emerged from the tapes, including claims from Al-Qaeda's number two, Ayman Al-Zawahiri that Tanweer and the suspected ringleader, Mohammed Sidique Khan, were trained by the terror network in Pakistan.
It is thought to be the first time a member of Al-Qaeda has made specific claims that the two bombers visited one of its bases.
But the police commissioner said the video was not sufficient proof that Al-Qaeda was directly responsible for the London bombings, and that an exhaustive investigation into its origins is still very much underway.
The King's Cross commemoration was led by London Mayor Ken Livingstone and Culture Secretary Tessa Jowell, who laid wreaths at the moment of the first bombing - 8:50 a.m. - then bowed their heads in tribute to the dead.
Many commuters paused to remember and to lay wreaths, not only at King's Cross but at the bomb sites -- the Aldgate, Edgware Road and Russell Square underground stations and at Tavistock Square, where the packed red double-decker bus was struck.
Memorial plaques were unveiled at all the sites shortly before noon.
In the afternoon, bereaved families and survivors were invited to visit the bomb sites and to attend a private service at St Ethelburga's Church in the City of London financial district.
But it was a 30-minute memorial service in Regent's Park that was the most painful for those caught up in what Londoners refer to as "7/7".
More than 1,000 people attended, including Mr Blair, political leaders and a representative of the royal family, and heard music, poems, tributes and songs, plus a roll call of the victims.
Families and survivors then completed a floral tribute - 12 metres wide, in the shape of a flower with seven petals - by placing yellow gerberas in the centre.