More than 6,000 police officers, many armed, are deployed to patrol London's underground stations and other transport networks on Thursday, four weeks after the first bombings hit the British capital in which 56 died and more than 700 were injured.

"We are using all the resources we can possibly muster to reassure Londoners and to prevent another attack," said Deputy Chief Constable Andy Trotter, quoted by the British Broadcasting Corporation (BBC).

Although police have received no intelligence about another attack, all leave has been cancelled and detectives have been drafted into uniform.

Extra officers are also brought in from outside the capital and retired officers persuaded to return to help with the anti-terror work.

"It's extremely costly, there is no doubt about it. The budget is under enormous pressure and operationally there is pressure," said Trotter, adding however, "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 said earlier that work on major murder investigations had "slowed to a trickle" because so many specialist detectives were working on anti-terror operations.

The high-visibility presence aims to make people feel safe while undercover officers mingle with commuters on the underground stations and buses trying to spot potential bombers.

Nonetheless, some 60 percent of people polled by Sky news said the public transport is no longer safe.

Source: Xinhua