Police have been out in force in London two weeks after the failed 21 July bombings and four since the 7 July attacks in which 52 people were killed.
Armed police patrol London's streets and transport network
More than 6,000 officers, many armed, have been patrolling stations and key sites, as the Piccadilly Line reopened.
"We are using all the resources we can possibly muster to reassure Londoners and to prevent another attack," said Deputy Chief Constable Andy Trotter.
The first person charged in connection with the attacks has appeared in court.
A high-visibility police presence in the capital was aimed at making the public feel safe, while undercover officers are mingling with passengers on Tubes and buses trying to spot would-be bombers.
Although police had received no intelligence about another attack, all leave was cancelled and detectives drafted into uniform.
Extra officers were also brought in from outside the capital and retired officers persuaded to return to help with the anti-terror work.
Detective Chief Constable Trotter said: "It's extremely costly, there is no doubt about it. The budget is under enormous pressure and operationally there is pressure.
"But this is the biggest threat London has faced in peacetime and we have to throw all our resources into it right now. We will continue as long as it is necessary."
Metropolitan Police Assistant Commissioner Tarique Ghaffur earlier said work on major murder investigations had "slowed to a trickle" because so many specialist detectives were working on anti-terror operations.
In the Leeds suburb of Beeston, police on Thursday handed out leaflets asking for information about the 7 July suicide bombers - the men all had links to the area.
Aldgate bomber Shehzad Tanweer, 22, was from Beeston; Edgware Road bomber Mohammad Sidique Khan, 30, lived there before moving to Dewsbury; bus bomber Hasib Mir Hussain, 18, was from Holbeck, Leeds.
Russell Square bomber Germaine Lindsay, 19, was from Aylesbury, Buckinghamshire, but spent his teens in Huddersfield, West Yorkshire.
London Underground said all stations on the Tube network were open from Thursday, with the Piccadilly Line reopening and the Circle Line beginning a limited service.
"It's a very poignant moment today," said Detective Chief Constable Trotter.
All Tube stations were open for the first time since 7 July
"Today is part of getting London back to normality. It will be a different normality from before - the level of security will have to change for a long, long time to come."
He added: "Obviously people are nervous and anxious, but they are happy to see us."
The Piccadilly Line had been suspended from Hyde Park Corner to Arnos Grove and from Rayners Lane to Uxbridge, and the Circle Line shut completely since 7 July.
The Hammersmith and City Line, also shut since 7 July, reopened on Tuesday.
Ismael Abdurahman, 23, of Newport Street, Kennington, south London, was remanded in custody at Bow Street Magistrates Court until 11 August.
Arrested on 28 July, he is charged with failing to disclose information about suspected Shepherd's Bush bomber Hussain Osman.
There are a further 14 people being held in connection with the attacks.