All four men suspected of involvement in the July 21 attacks on London's transit system were believed to be in police custody Friday after a rapid-fire series of dramatic events that spanned the continent.

Two of the suspects were picked up in a west London apartment complex following raids by heavily armed police that sent residents scurrying out of their homes and away from the scene.

"The men in the premises were asked to surrender but failed to do so. As a result, in order to minimize potential risk to the public and to the police officers, specialist tactics were used," said Peter Clarke, head of London's Metropolitan Police (search) counter-terrorism unit.

One of the men identified himself as Muktar Said Ibrahim (search), 27, who allegedly tried to blow himself up on a double-decker bus in east London. The other man said his name was Ramsi Mohammed (search), who is believed to have been responsible for the attempted bombing near Oval Tube station.

He was pictured running from the Oval Street subway station wearing a "New York" sweatshirt.

Italian police in Rome arrested Osman Hussain (search), a naturalized British citizen from Somalia, as part of an ongoing investigation in the bombings, said Interior Minister Giuseppe Pisanu. Hussain, who reportedly fled London and stopped in Milan and Bologna en route to the Italian capital, was "the fourth attacker," he said.

Hussain is suspected of targeting a subway train near the Shepherd's Bush station. He was shown on closed-circuit TV footage wearing a backpack in the Westbourne Park station. Clarke said that British authorities would seek to have him extradited from Rome.

Hussain was arrested in Rome after police traced his cell phone calls across Europe after the attacks, officials and media reports said.

With those arrests, as well as that of Yasin Hassan Omar (search) on Wednesday in the city of Birmingham, authorities believe they have captured all four men whose photos they released following last week's botched bombings, a police official said.

Clarke apologized and thanked Londoners for bearing with the raids, and warned that there would be "more very visible police activity. I cannot set out where and when this might be, but I would emphasize the need for the continuing support and understanding of the public as these operations continue."

No one died in last week's attacks due to what police believe to have been blunders by the bombers, but the timing, so close to the devastating July 7 transit bombings that killed 56 people, have the city wondering if there's an end in sight.

Clarke urged the public to stay on guard, but also sought to reassure people that police were working around the clock to prevent more attacks.

"Despite the progress we must not be complacent. The threat remains and is very real. The public must be watchful and alert," he said.

"We are doing all we can to keep you safe."

So far, about two dozen people have been arrested in connection with the attacks last week in which bombs in backpacks failed to detonate on three subway trains and a double-decker bus. The deadly July 7 attacks also killed the four homicide bombers.

The Notting Hill raids took place near Portobello Road in the chic neighborhood famous for its weekend street market. The area is near west London's Little Wormwood Scrubs park, where police on Saturday found a fifth bomb in a dark backpack.

Helicopters buzzed overhead in the area and police cordoned off a number of streets. The operation began around 11:30 a.m. local time in London.

There were reports of at least eight blasts in the area. Those blasts could have been the sound of stun grenades or gas canisters being used by police to gain entry into the building.

Sky News on Friday broadcast video of two of the men arrested in light blue body suits designed to preserve evidence leading away a man in a white bodysuit, shielding his face.

In addition, a witness told The Associated Press that a man wearing what appeared to be a bus driver's uniform was led away by police in handcuffs.

The witness, Osama Ahmed Ali, saw a Somali man whom he recognized as a bus driver.

"He was in a purple-and-yellow bus driver uniform," said Ali, 16. "I've been on a bus with him a couple of times."

Police also arrested two women at the Liverpool Street train station in central London and evacuated the area. The women were pinned to the ground in a raid.

One woman is believed to have been in a line for the Stansted Express, which goes to one of London's airports, when she was pushed to the ground by police.

The two women were arrested at 1:54 p.m., British Transport Police said. The police were searching a number of suspect packages in the station.

Some Notting Hill residents said they initially mistook the police raids for another terrorist attack.

"I heard six loud bangs, which I found out from a policeman were stun grenades I believe, and then I heard two shots," witness Patrick Ball said. "The noise that I heard was an extremely loud bang."

The 24-year-old Omar is a Somali citizen with British residency. Omar, suspected of attacking the Warren Street subway station, was being questioned at a top-security police station in London.

Meanwhile, a police watchdog group investigated the killing of a Brazilian electrician by police, who believed he was a suicide bomber.

Investigators from the Independent Police Complaints Commission appealed for witnesses who were at Stockwell subway station in south London on July 22, when Jean Charles de Menezes, 27, was shot eight times — including seven times to the head.

Menezes' funeral is being held Friday in Gonzaga, Brazil, where he was born. Cardinal Cormac Murphy-O'Connor, leader of the Roman Catholic Church in England and Wales, will attend a Mass for Menezes at Westminster Cathedral in central London Friday evening.

One of the Tube stations closed after the July 7 attacks reopened Friday. Several bouquets of flowers lay at the entrance to the Edgware Road station in a tribute to the seven people killed there. But passenger numbers were visibly down — a sign of nervousness among Londoners despite a huge police operation to catch the terrorists.

"I felt a bit nervous coming through the tunnel just then and this morning my mum gave me a look as though she was never going to see me again," commuter Jasmine Chandhoke, 22, said. "Everyone was being incredibly vigilant on the train, checking each other's bags."

Scotland Yard police headquarters declined to comment on the arrest in Zambia of a British man sought in connection with the July 7 bombings.

British investigators reportedly believe Haroon Rashid Aswat, 31, had been in telephone contact with some of the four suicide attackers who carried out the July 7 attacks. Aswat told investigators he once was a personal guard for Usama bin Laden, Zambian security officials said on condition of anonymity because they are not authorized to speak publicly.

The British Foreign Office said it was seeking access to a Briton reportedly detained in Zambia but would not identify him.

Steve Purl, a former Scotland Yard (search) team leader, said police likely used explosive entry to get into the building, but "for whatever reason, they can't go any further...

"This clearly is going to be the tip of the iceberg and in the next days, week, possibly months, we're going to see a lot of activity in the London area," Purl said.

Purl said any gas canisters that may have been thrown inside the building consist of high-decibel sounds aimed at disorienting people inside. He said using gas to gain entry in such situations is "somewhat extreme."

"It looks most professional … it's a remarkable set of developments for Britain," former CIA Director James Woolsey (search) told FOX News. "It looks like the Brits are turning on the heat and 'bravo.'"

FOX News' David Lee Miller and The Associated Press contributed to this report.

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