British police are continuing to question three men and a woman in connection with the suicide attacks in New York and Washington.
Two men, aged 27 and 29, and a 25-year-old woman were arrested at houses in west London yesterday.
Another man, in his mid-40s, was detained in Birmingham and was taken to London for questioning.
The arrests were the first to be made in Britain in connection with the September 11th attacks on New York's World Trade Center and the Pentagon in Washington.
Police in England and Wales can hold suspects for up to 48 hours on their own authority under the terms of the new Terrorism Act legislation passed last year.
After two days police must apply to magistrates for permission to hold suspects for further periods of up to seven days, after which they must charge or release them.
Metropolitan Police Commissioner John Stevens said yesterday that London was on the highest peacetime security alert in its history, with 1,500 extra officers on the streets this weekend.
London is at risk because it is one of the world's major capital cities, although there has been no specific threat, he said.