The city's transit system took a step toward normality with the reopening of the subway's busy Piccadilly Line.
Officials stressed there was no specific intelligence of a third attack on Thursday, but undercover police were mingling with passengers, and officers armed with automatic rifles and pistols patrolled stations and streets. Police helicopters hovered above, and traffic was heavier than normal.
Police said the massive security operation, involving some 6000 officers, was intended to reassure the public four weeks after the deadly 7 July attacks and the failed 21 July attacks that followed.
"It's a little bit eerie," said Rosalyn Cooper, 23, before she got on the Tube. "You can't help thinking about when it will happen again."
Police said they wanted to
reassure the public
"This is a major threat and we are doing everything we can, both covert and undercover policing, to try to stop another attack," said Deputy Chief Constable Andy Trotter of the British Transport Police.
"There is no specific intelligence but we are still at a very high level of alert in London. It's four weeks on from the first attack, and this operation today is to reassure Londoners."
Meanwhile, the first person charged in connection with the failed 21 July bomb attacks appeared in court on Thursday and was ordered held until his next hearing in a week.
In the first court hearing in Britain in connection with the attacks, Ismael Abdurahman, 23, from southeast London, spoke only to confirm his name and age.
He was not asked to enter a plea during a 10-minute hearing in Bow Street Magistrates' Court before being led off to jail. He is scheduled to appear in court next on 11 August.
He faces charges of withholding information that helped suspected subway bomber Hamdi Issac initially avoid capture. Issac, suspected of trying to blow up a subway train on 21 July, was later detained in Rome, and is being held there on international terrorism charges.
Britain has requested that Issac, also known as Osman Hussain, be extradited from Italy. His lawyer, Antonietta Sonnessa, said an extradition hearing was set for 17 August.
Britain is holding 14 suspects in connection with the investigation, and Italy has three. Police believe they have all the 21 July bombers in custody.
There have been no arrests in connection with the 7 July attacks, in which the four bombers are believed to have died.