A terror attack on London is inevitable, Metropolitan Police commissioner Sir John Stevens has said.
The police chief was speaking in central London
Sir John's comments were echoed by the capital's mayor Ken Livingstone who said it would be "miraculous" if London escaped attack.
The two men were speaking after last week's terror attacks in Madrid which left 200 dead.
Earlier cabinet minister Peter Hain said the UK was a "frontline target" for attack by international terrorists.
Job to stop attack
Speaking at a mayor's press conference at City Hall, Sir John said UK security services were working "three times harder than ever" in a bid to foil possible attack.
"Since 11 September there have been 520 arrests, half have been charged with an offence and there are 90 about to go through to court," Sir John said.
"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."
Mr Hain, who is leader of the Commons, said anyone resisting international terrorism became a target.
Referring to the threat of attack on the UK he told BBC Radio 4's Today programme: "Of course we are all frontline targets [of terrorism] - that's why we have been adopting the preparations we have now for over a number of years.
"The point I am making is that these terrorists will strike against anything that threatens their extremism."
On Monday Home Secretary David Blunkett urged people to be "alert but not alarmed", although it was "quite likely" a terror attack was being planned now against the UK.
His comments came as plain clothes anti-terror police began patrolling London's public transport system.
British Transport Police also plan more random searches for Tube passengers.
A spokesman stressed the moves had been planned for some time and were not a reaction to the attacks in Madrid.
On Monday Mr Blunkett told BBC News: "We are at no greater risk now than we were before the World Trade Center attack, because they had already planned for it, before the Afghanistan war, before the Iraqi conflict and before last Thursday."
Ministers had stressed they could not guarantee there would never be an attack, he said, adding: "It is quite likely they are planning one now."
On Monday UK premier Tony Blair held his first telephone conversation with Spain's new socialist prime minister-elect Jose Luis Rodriguez Zapatero.
Mr Zapatero - who was against the war in Iraq - won power on Sunday.
Mr Zapatero used strong language in criticising the war
Mr Blair enjoyed a close relationship with Mr Zapatero's predecessor, Jose Maria Aznar who supported the invasion of Iraq.
The socialist leader has already attacked the US and UK saying that Mr Blair and George Bush need to engage in some self-criticism in the wake of the war - which he described as a "big disaster".
Mr Hain said he believed that the UK and the rest of Europe was united by common objectives for both the EU and for Iraq's future - despite differences over last year's war.