A terrorist attack on Britain is inevitable, the Metropolitan Police Commissioner Sir John Stevens said today.
Police and the security services have dramatically increased their efforts and are "working three times harder than ever" to foil any terrorist plot.
London Mayor Ken Livingstone told a City Hall press conference that it would be "miraculous" if the capital was not attacked.
Sir John said: "Since September 11 there have been 520 arrests, half have been charged with an offence and there are 90 about to go through to court.
"We do know that we have actually stopped terrorist attacks happening in London but, as the Prime Minister and Home Secretary have said, there is an inevitablity that some sort of attack will get through but my job is to make sure that does not happen."
Sir John's comments came amid mounting evidence that Islamic extremists with links to al Qaida were behind last week's Madrid bombings that left 201 people dead.
An extra 680 officers have been drafted in for anti-terrorism operations and a budget of £52 million for 2004/2005 was expected for the Metropolitan Police.
London remained on a high state of alert and everyone needed to be vigilant, said Sir John. He said: "We are not talking about the Tube and the rail system. We are talking about London generally. We are talking about buses, anything seen of suspicion in clubs. This is a general request to be alert."
But he urged the public not to panic and to go about their normal lives, adding: "A major catastrophe in London will not be easy to handle. None of them, are but we have major experience over the last 32 years and we will take action."
He continued: "If terrorists do get through and commit an offence, we must make sure we arrest them and deal with them as soon as possible."
Mr Livingstone said: "It would be miraculous if, with all the terrorist resources arranged against us, if terrorists did not get through, and given that some are prepared to give their own lives, it would be inconceivable that someone does not get through to London."
He added: "We have known all along there are people trying to come here and take lives. Everything we have done has been simply to protect life. There are people out their who will take life in the hundreds and thousands."
Mr Livingstone said that London remains one of the safest cities in the world. He pointed out that most venues are covered by CCTV cameras which would make it difficult for bombers to escape. And cameras used for transport systems such as the congestion charge could also be used to close the net on terrorists.
Sir John said the Met were looking closely at the events in Madrid. He said that the Met have sent a team to Spain who are "there to learn" from what happened. They are not investigating last week's bombing, he stressed.
* Three-quarters of Britons feel "more vulnerable" to terrorist attack because of the Government's decision to join military action in Iraq, according to a poll released today. Almost half of those questioned (48%) said Britain was still right to go to war.
The survey for Sky News was conducted just days after Spain's Popular Party administration was swept from power amid opposition claims that its support for military action in Iraq made the country into a target for al Qaida.
Results of the survey suggest the public is not persuaded by the argument, set out by ministers including Foreign Secretary Jack Straw after last week's train-bombs in Madrid, that the war had not made Britain less safe.
There was little indication that the Madrid atrocities had affected voting intentions in the UK, with the YouGov poll putting the Conservatives on 39%, Labour on 34% and the anti-war Liberal Democrats on 20%.
This was the same result recorded by the company in a similar poll at the end of February.
If there was a terror attack in Britain, some 20% of people who voted Labour in 2001 said they would be likely to switch to another party, with the majority naming the Liberal Democrats.
Eight per cent said they would be more likely to vote Labour following an outrage.
Asked if they felt Britain's involvement in the Iraq war had made the UK a target for Islamist terrorists, some 75% said they felt more vulnerable. One per cent said they felt safer and 22% said it had made no difference.
Forty-eight per cent said Britain and the United States were right to take military action, against 41% who thought they were wrong. A large majority - 73% - said the prospect of terrorist attacks was unlikely to make a difference to the way they would vote in the general election expected next spring.
The Government won a vote of confidence in its handling of the terror threat, with 55% of respondents saying it had done a good job, against 33% who rated it as poor.Reuse content