Anti terror police patrol London Tube

LONDON - Plain clothes anti-terrorist police are patrolling London's underground rail network, as officers urge travellers to be extra-vigilant after last week's multiple bomb attacks on Madrid trains that killed 200 people.

Under new security guidelines, police will be able to stop and search passengers they think may be acting suspiciously.

Deputy Chief Constable Andy Trotter, of the British Transport Police, said that while there had been no specific threat against London Underground or the capital's rail network, "the threat from terrorism remains very real, as the events in Madrid show".

"London is on a high level of alert and the London Underground system is controlled public space, with hundreds of staff at stations, extensive CCTV coverage and its own dedicated police resources," he said.

"A further, crucial element is the vigilance of customers. With three million travelling every day, they can provide millions of eyes and ears across the whole system."

Police said the initiative had been put in place before Thursday's attack on the Spanish capital and was not connected, but with speculation mounting that al Qaeda was to blame, focus was turning to the risk to other Western cities.

"The despicable attacks in Madrid show only too clearly that the threat from terrorism remains very real. London continues to be on a high level of alert and security across the capital remains under constant review," said Peter Clarke, head of the Metropolitan Police's Anti-Terrorist Branch.

"We will do whatever is necessary to protect the public from harm. Public vigilance is vital at all times, not only when travelling on the Tube or rail network."

Police are appealing to passengers to report anything suspicious, query other passengers if they see unattended luggage and report any unclaimed bags.

Posters driving home the safety message will go up on the underground network this week.

The security measures come after experts from underground systems across the world met in London last week to review methods of combating terrorism.