A 27-year-old man and a 25-year-old woman were arrested at a residence in west London at about 3 a.m., a Scotland Yard spokesman said on customary condition of anonymity. A 29-year-old man was arrested at a separate location in west London.

Officers searched both sites using a warrant issued under a British terrorism law passed last year, the official said.

At about 7 p.m. a fourth suspect, a male whose age was not disclosed, was detained at an address in Birmingham, 100 miles north of London.

''They were arrested in connection with the World Trade Center terrorist attack, and are being questioned by anti-terrorist branch officers,'' the spokesman told the Associated Press, reading from a statement.

''They were taken to a central London police station where they remain in custody.'' No charges have been filed.

The spokesman refused to provide any more information on the suspects or elaborate further on any connection between them and the U.S. attacks.

The arrests came after Britain's most senior police officer, Metropolitan Police Commissioner John Stevens, said that London was on its highest security alert ever during peacetime, with 1,500 extra officers on the streets. He said there was no specific threat against London.

The police spokesman said Scotland Yard was working with the FBI on investigating the terrorist attacks that have left thousands dead.

The arrests came as investigations into the terror attacks continued in other European countries.

French authorities arrested seven people Friday in connection with an alleged plot against U.S. interests in France. German authorities announced they were seeking two fugitives believed to have helped plot the four deadly hijackings in the United States.

It is estimated that between 200 and 300 British nationals were killed in the attacks on the World Trade Center, although Britain's Foreign Office said Friday no British bodies had yet been recovered from the ruins.

British Prime Minister Tony Blair has said Britain is fully backing America's war against terrorism. Friday, he completed a four-nation trip to strengthen support for military action against those responsible for the Sept. 11 suicide hijackings.

Blair's whirlwind mission ended in Brussels, with the EU agreeing to a package of measures designed to combat international terrorism and a statement pledging support for a strike against those directly responsible for the tragedies.