LONDON, March 16 (Xinhuanet) -- London police authorities Tuesday warned that a terror attack on Britain's capital city is inevitable and said security services are working "three times harder than ever" in a bid to foil possible attacks.

    "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 inevitability that some sort of attack willget through but my job is to make sure that does not happen," saidMetropolitan Police commissioner John Stevens.

    Stevens'comments at London Mayor Ken Livingstone's press conference at the City Hall Tuesday morning were shared by Livingstone who said it would be "miraculous" if London escaped attack in the wake of last week's train bombings in Madrid which left 200 dead.

    The police chief said: "Since 11 September there have been 520 arrests, half have been charged with an offense and there are 90 about to go to court."

    On Monday Home Secretary David Blunkett told BBC News it was "quite likely" that a terror attack was being planned now against Britain. He urged people to be "alert but not alarmed."

    "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," Blunkett said.

    Plainclothes anti-terror police started patrolling the London underground for the first time on Monday while uniformed officers have begun random stop-and-search operations.

    Meanwhile, posters are being put up to encourage public vigilance with underground passengers being warned to make their own checks for unattended baggage.

    On Sunday, Defense Secretary Geoff Hoon said the Madrid train bombings highlighted the need for passenger vigilance.

    He urged people to report "suspicious and unusual" things to the police. "If there are any packages, any briefcases and suitcases that don't appear to belong to any particular individualthey should ask each other," he said. Enditem