British police Friday arrested three people suspected of involvement in last week's attempting bombings of London's transportation system, and Italian authorities picked up another suspect they identified as one of the would-be bombers.

The raids in London and Rome, combined with the arrest on Wednesday of a suspect in Birmingham, have now netted all four men suspected of trying to bomb three subway trains and a double-decker bus in London on July 21, British news media reported.

In an evening news briefing at Scotland Yard, Peter Clarke, head of the police antiterrorist branch, confirmed the arrest earlier Friday of Muktar Said Ibrahim, 27, a naturalized British citizen originally from Eritrea who police say tried to blow up the No. 26 double-decker bus in last week's abortive attacks. Arrested with him, Clarke said, was Ramzi Mohammed, a suspect whose name had not previously been made public.

Clarke did not identify a third man who was arrested in a separate raid in west London. But he confirmed that Italian police, acting on a European arrest warrant, picked up a suspect he identified as Hussain Osman, and he said Britain would seek his extradition.

Clarke said the fast-moving investigation is continuing, and he warned that Britain still faces the threat of more terrorist attacks.

"Despite the progress that has been made with the investigation, we must not be complacent," he said. "The threat remains and is very real."

British media said that in addition to Ibrahim, who allegedly tried to bomb the No. 26 bus, the raids in west London Friday caught the man who carried a backpack aboard a subway train at London's Oval station. In a raid in Birmingham Wednesday, police arrested Yasin Hassan Omar, a 24-year-old Somali immigrant who allegedly tried to bomb a subway train at the Warren Street station.

In Rome, Italian Interior Minister Giuseppe Pisanu said the man arrested there is a 27-year-old British national of Somali origin, who was shown on British surveillance footage carrying a backpack and wearing a blue baseball cap in the Shepherd's Bush subway station. Pisanu gave his name in a different order from the British, identifying him as Osman Hussain. It was not immediately clear how he reached Rome, but Italian news media reported that he traveled through Paris and Milan.

An Interior Ministry official said Osman has a brother-in-law in Rome and was tracked through a cell phone that the relative had given him. The official said the suspect was arrested without resistance at an apartment owned by an illegal immigrant. Antiterrorist police who broke into the apartment did not expect to find Osman there and were surprised to find the cell phone, whose number was provided by British authorities, still in his possession, the official said. An antiterrorist operation was continuing, and police were searching for other suspects, the official said.

British police announced this week that a bomb similar to the four that failed to detonate was found unexploded in a backpack in west London, leading to speculation that a fifth would-be bomber was at large.

Heavily armed police, including SWAT teams equipped with gas masks, tear gas and stun grenades, arrested Ibrahim and Mohammed at the Peabody Buildings, an apartment complex in west London, and another man at a residence in the upscale Notting Hill neighborhood, police said.

Clarke said Ibrahim and Mohammed initially refused to surrender and that as a result, police employed "specialist tactics" to arrest them. He said the premises will now be thoroughly searched.

British news media identified one of the men arrested in London Friday as the suspect who was shown on a surveillance camera running from the Oval subway station in a dark sweatshirt with a "New York" logo after last week's attempting bombings.

Police did not immediately confirm this. Clarke declined to answer questions after the news briefing.

British television news footage showed heavily armed SWAT team members arresting a man who appeared to be Ibrahim and another man on an apartment balcony after apparently having fired tear gas into the apartment. Both men appeared on the balcony stripped to the waist and were ordered to put their hands on their heads.

Police have said the attacks last week were apparently meant to replicate the devastation of July 7, when four suicide bombers blew up three subway trains and a bus, killing 52 other people and injuring 700. However, all four of the July 21 bombs failed to explode, and the attackers ran from the scenes.

Residents of the area where Friday's arrests were made told reporters they heard what appeared to be stun grenades and gunshots, which may have been the firing of tear gas canisters, as police carried out the raids.

One woman said police tried to negotiate with one of the men for more than 30 minutes before storming the apartment in which he was hiding. She said she heard police order the man to come out wearing only his underwear so that they would know he was not wearing explosives.

She said in a television interview broadcast on CNN that the suspect, sounding scared and possibly crying, asked, "How do I know you won't shoot me?" After the suspect remained silent for several minutes, she said, she heard small explosions, and the suspect was taken into custody.

A man who lives nearby said he heard six loud bangs that he took to be stun grenades, following by two shots.

The British television network Sky News broadcast footage of two police officers in light blue body suits leading away a man wearing a white body suit and shielding his face.

The Associated Press quoted a witness as saying one of the men arrested appeared to be wearing a bus driver's uniform as he was led out in handcuffs.

In a separate raid at the Liverpool Street subway station in central London, armed police arrested two women and evacuated the area after forcing the women to lie on the ground.

More than two dozen people have been arrested so far in an investigation of the latest bombings.

Branigin reported from Washington. Special correspondent Bill Magnuson in Rome contributed to this report.