One year after four young British Muslims blew themselves up on London's transport system, killing 52 people and wounding 700, the mood was one of unease mixed with resignation and defiance.
Ian Blair, the Metropolitan Police Commissioner, described the threat faced by Britain as "grim".
"There are, as we speak, people in the United Kingdom planning further atrocities," he told BBC Radio. "Since July, the threat has palpably increased."
In what appeared to be a well-timed attempt to fuel fears, a video surfaced on Thursday, apparently showing one of the London bombers, Shehzad Tanweer, reading his last testament.
"There are, as we speak, people in the United Kingdom planning further atrocities"
Ian Blair, Metropolitan Police Commissioner
The previously unseen video, broadcast on Aljazeera television, also showed al-Qaeda's deputy leader Ayman al-Zawahiri, suggesting a link between Osama bin Laden's network and the bombers.
The failure of the police to charge anyone in connection with Western Europe's first suicide attack has added to unease.
The British government says it does not know much about the motivation of the bombers, their possible training abroad, or their alleged links to al-Qaeda.
A survey by the Pew Global Attitudes Project, released in Washington on Thursday, indicated that 42% percent of the British people were very concerned about a rise in Islamic extremism in Britain compared with 34% a year ago.
Many of Britain's 1.8 million Muslims feel that the police have been unfair to their community since the attacks.
Two unsuccessful anti-terrorist operations in which police shot two innocent men, killing one of them, have not helped.
Tony Blair, the British prime minister, whose decision to send troops to Iraq and Afghanistan was cited by Tanweer as a reason for the bombings, did not address such criticism in a statement to mark the occasion.
Al-Qaeda was said to be linked to
"Today is a day of remembrance across London and the United Kingdom," said Blair, who joined the nation in a 2-minute silence at midday (1100 GMT).
"It is a chance for the whole nation to come together to offer comfort and support to those who lost loved ones or were injured on that terrible day."
Earlier, candles of remembrance were lit under the vast dome of St Paul's Cathedral at 8.50am and at 9.47am, the times the bombs went off, as a single bell tolled.