"We must not be complacent," he said. "The threat remains and is very real."

The earlier statement from Rome was the first indication that a suspect had gotten as far as Italy. One unconfirmed report after the failed July 21 bombings said one would-be bomber had fled to Belgium or the Netherlands. Another said the man had left England in the past two or three days and had been tracked through Paris and Milan by cellphone calls to a brother-in-law resident in Rome.

But the announcement from Rome seemed certain to raise questions about how a failed bomber had managed to slip out of Britain undetected, particularly since images showing all four suspects on closed-circuit television were made public just a day after the July 21 attacks.

In London today, daylight raids on streets in Kensal Green and Westbourne Park injected what has become familiar imagery in this city under siege since the first terror attack on July 7 - streets cordoned and residents fleeing homes as the police ring unremarkable homes and apartments in quiet streets. People fled in underwear and dressing-gowns, clutching children, shoes and clothes, chased from their homes at late morning.

A separate police statement today said two women, not identified by name, were arrested under anti-terrorism laws at Liverpool Street mainline station. The area around the station was sealed off after one of the women tried to run after refusing a police order to open a bag, witnesses said.

In west London, explosions, possibly from stun grenades, boomed across leafy streets as police moved in, and at least one suspect was seen on television being led away handcuffed and clad in a white head-to-toe coverall to prevent the loss of forensic evidence.

Police snipers using rifles with telescopic sights took up position overlooking the apartments where the arrests happened. The raids were arranged at short notice, probably after informants responded to increasingly desperate police appeals for information leading them to the suspects.

Television showed police ordering one man, whom they addressed only as Mohammed, to undress and surrender.