Police commandos firing tear gas and stun grenades on Friday seized two suspected bombers in London, and authorities in Rome arrested a third, in a dramatic conclusion to the hunt for the four men wanted in last week's failed attacks on London's transit system.

Police officials, who already had one man in custody and feared the others would strike again, were jubilant over the breakthrough. The arrests appear to give them a rare prize: a full cell of alleged terrorists captured alive and unharmed, who could be questioned about possible links between their abortive attacks and the July 7 bombings that killed at least 56 people, including four presumed bombers.

In a day of fast-moving developments, police arrested another man in London's fashionable Notting Hill neighborhood and are investigating whether he was responsible for an unexploded fifth bomb that was found abandoned in a park.

Police remain uncertain as to who else might have aided in the plot. Peter Clarke, head of the counterterrorism branch of London's Metropolitan Police, maintained a cautionary tone after the arrests. "We must not be complacent," he warned in a televised statement. "The threat remains, and is very real. The public must be watchful and alert."

The four suspects are alleged to have carried explosives onto three trains and a bus on July 21 in what investigators called an attempt to replicate the carnage of July 7. But all four bombs failed to explode, and the men fled the scene.

An enormous manhunt ensued, with police blanketing Britain with security camera images of the suspects. One of them, Yasin Hassan Omar, 24, was captured Wednesday in Birmingham. He is accused of trying to blow up a subway train near the Warren Street station.

Friday's drama began late in the morning when police ordered residents of the tranquil, leafy Ladbroke Grove area of West London to evacuate their homes. Dozens of heavily armed officers in flak jackets and chemical suits surrounded an apartment in a red-brick complex and ordered the people inside -- whom they considered potential suicide bombers -- to surrender.

"You must do what we say!" an officer commanded. "You will be safe if you do what you're told!"

"I have rights! I have rights!" a man shouted back, in an exchange that was captured by Lisa Davis, a neighbor, on her cell phone and broadcast on Sky News.

"Mohammed, we've told you to leave twice!" another officer shouted. "Now I want you to leave the flat!"

At one point, the man shouted: "How do I know you're not going to shoot me?"

The siege lasted 20 minutes. At one point, two small children emerged from an apartment one floor below to get a closer look at a police dog and were scooped up by officers and rushed away. At the same time, police officials pleaded with television networks to stop live broadcasts from the scene out of concern the suspects would watch and see the officers' locations.

When the man inside the flat stopped responding, an officer demanded, "Why don't you come out, Mohammed? Is there anybody in there that's stopping you coming out?"

Within seconds, witnesses heard several large bangs -- identified by observers later as stun grenades -- and police stormed the apartment firing tear gas. Minutes later, two dazed-looking men emerged on the balcony, stripped to the waist with their hands up. Footage broadcast on the ITV network showed the two men wincing, blowing their noses and spitting, apparently from the effects of the gas.

One was bundled into a hooded, white forensics suit, while the other was dressed in street clothing. Both were then rushed to nearby police vehicles and taken away for interrogation.

Police officials later identified the two as Ibrahim Muktar Said, 27, and Ramzi Mohammed. The officials had previously named Ibrahim as the man who attempted to detonate a backpack full of explosives on a double-decker bus in East London last Thursday. Officials said they believed Mohammed was the man who tried to detonate a similar bomb aboard a subway train near the Oval station in South London at the same time.

Hours later, Italian Interior Minister Giuseppe Pisanu announced that police in Rome had arrested Osman Hussain, 27, a British citizen of Somali origin who was suspected of the failed bombing of a subway train outside the Shepherd's Bush station in West London.

Two anti-terrorism units of the Italian state police burst into the apartment of Hussain's brother-in-law, according to an Interior Ministry official, who said Hussain did not resist.

According to Italian media reports, investigators located Hussain by tracking electronic signals from his cell phone or actual calls he made. That probe showed that he was first in London, then Paris, then Milan and finally Rome.

Hussain's brother-in-law, who operates an Internet cafe and cell phone service, was also detained for questioning.

"Today has been a day of intense activity," said Clarke, of the Metropolitan Police. "The investigation continues to develop at great speed."

In the evening, after Clarke spoke, armed police raided a house in South London.

Besides the four suspects, the authorities are holding 20 people for questioning, some of whom are suspected of having helped the alleged bombers to hide in the days after the July 21 attacks. The British media reported that on Friday officers pushed to the ground and arrested two women who were buying tickets for a train to London's Stansted Airport. It was unclear what, if any, connection they had to the investigation.

Because the four alleged bombers have been captured alive, they could help investigators answer many questions -- including whether there was a link, as investigators suspect, between the July 21 abortive attacks and the deadly July 7 blasts; who ultimately planned and directed the attacks; whether the al Qaeda network was involved; and how Hussain managed to escape Britain.

"There are so many things we still don't know," said a police official who spoke on condition of anonymity because of the sensitivity of the investigation.

Meanwhile, the Brazilian man gunned down in a subway car July 22 by police who mistook him for a suicide bomber was buried before thousands of people Friday evening in Gonzaga, Brazil, his birthplace. Mourners in London attended a memorial mass led by Cardinal Cormac Murphy-O'Connor, leader of the Roman Catholic Church in England and Wales.

The cardinal said Jean Charles de Menezes's family and friends deserved a full investigation into the killing of the 27-year old electrician, who was shot seven times in the back of the head and once in the shoulder by an undercover officer following "shoot-to-kill" rules for dealing with suicide bombers. "Britain must also learn the truth," the cardinal added. "We must learn the right lessons from the death of this innocent young man."

Meanwhile, Gen. Pervez Musharraf, Pakistan's president, ordered all foreign students at madrassas, or religious schools, to leave the country, according to wire service reports. The schools have come under renewed scrutiny after reports that at least one of the four alleged suicide bombers in the July 7 attacks became radicalized while studying at a madrassa.

The order is part of a new crackdown on Islamic extremism that Musharraf has ordered in the wake of the bombings. More than 200 people have been arrested, although none of the detainees are alleged to have had any role in the attacks. "We will not allow madrassas to be misused for extremism, hatred being projected in our society," Musharraf told foreign journalists in Islamabad.

Special correspondent William Magnuson in Rome contributed to this report.

In West London, two suspects appear, stripped to the waist, on the balcony of an apartment after a 20-minute siege by police. Officials identified the two as suspects in last week's abortive attacks.In the Notting Hill area of London, officers in coveralls shield the face of a suspect, center, as he is led away. Police are probing whether he was responsible for an unexploded fifth bomb on July 21. In Rome, a police officer inspects an apartment after one of the July 21 bombing suspects, 27-year-old Osman Hussein, was arrested at his brother-in-law's home there.Police in protective gear prepare to conduct raids in West London. Two suspects in last week's attacks were arrested. Video images show the four suspects sought and arrested in connection with the botched bombings in London on July 21. Police remain uncertain as to who else might have aided in the plot to blow up three trains and a bus.