BRITISH anti-terrorist police swooped on four people yesterday in connection with last week's attacks on the US.

A man, aged 27, and a woman, 25, were arrested at a residential address in west London at about 3pm. A second man, aged 29, was held at a separate address in the same area.

Officers executed a search warrant issued under the Terrorism Act 2000 at both addresses.

At about 7pm, a fourth person, a man, was arrested by anti-terrorist officers in Birmingham.

Scotland Yard said all four suspects were being questioned at central London police stations.

The arrests followed warnings that Britain should be on the "highest possible" alert this weekend amid growing fears of a second wave of terrorist attacks.

An extra 1,500 police officers will be on the streets of London as senior Scotland Yard officers warned the city could be the next target for Osama bin Laden's Al Quaida network.

In the US, the FBI is bracing itself for possible further disasters amid reports of terror suspects having booked seats on flights today.

An FBI spokesman said: "We can't rule out the possibility of further terrorist activity.

"We aren't saying anything specific about dates - we are just urging people to be cautions."

Tension rose yesterday when Afghanistan ruled out handing over bin Laden, making a US military operation against the ruling Taliban regime seem inevitable.

Britain's most senior police officer Sir John Stevens has warned that the next biggest target for the terrorists after the US would be London.

The Scotland Yard commissioner said there was no specific threat against the capital - but when asked who was the biggest ally to the US and what could be the next biggest target, he said: "It's got to be here."

Professor Paul Wilkinson, director of the Centre for the Study of Terrorism and Political Violence at St Andrews University, said the police, intelligence service and the public should be at the highest state of readiness.

He said: "If the Metropolitan Police Commissioner believes there is a real threat, I'm sure that this is something he believes on the basis of intelligence evidence about these groups.

"We certainly know that bin Laden's network has had people in the UK for some time and the authorities have discovered that some of the people involved in the hijacking spent time in Britain.

"This means that they have been able to move around in Europe and inside this country.

"They clearly regard all the allies of the US as a potential target and bin Laden has made that clear.

"As we tend to be a particularly close ally of the US we would be wise to to be on alert because we are not immune from these kind of attacks.

"We should be at a very high state of preparedness, the greatest possible effort should be put in to ensure the intelligence service, police and counter-terrorism capabilities are at the highest level of readiness.

"And we should have public opinion prepared for cooperation with the police.

"As we know from terrorism spilling over from Northern Ireland, it is often members of the public with information that helps to thwart terrorism. I don't think we can assume these are the last efforts we will see from this terrorist network."

He said there was a global threat to British personnel and facilities abroad as well as to cities here.

"We cannot now fix on Afghanistan because that's not the full picture," he said.

"We have to recognise there are cells operating far away from bin Laden's base.

"We have to remember that they have attacked embassies, shipping and military targets."

Fears of further attacks have also been fuelled by reports that four suspects linked to the 19 hijackers booked seats on United Airlines flights in the US today.

Two of them reportedly bought tickets for Flight 1429 from San Antonio to Denver and the other two for Flight 1185 from Denver to San Diego.