British police have made a second arrest in connection with an attempted bomb attack on London's (search) transit systems.

The suspect was apprehended in Stockwell, the south London neighborhood where another suspect was detained Friday and another man was fatally shot by police in a subway station, Scotland Yard (search) said.

Police say the man they shot dead in south London on Friday was not connected to the bombing investigation.

The first suspect, who has not been named, is being questioned at a high-security London police station. Meanwhile, police are mounting a massive manhunt for the terrorists who tried to bomb three subway trains and a bus on Thursday — two weeks after 56 people were killed in four homicide bombings (search) that also targeted the London transit system.

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The atmosphere remained tense Saturday as London police evacuated the Mile End station in the city's East End due to a "security incident."

One person at the station said there was the smell of burning rubber.

Service on parts of two subway lines was briefly suspended but a spokesman for London's Metropolitan Police said it "turned out to be nothing."

On Friday police released closed-circuit television images of four suspected attackers and appealed to the public for information.

"It's crucial that detectives are able to question them about yesterday's events," Andy Hayman, assistant Metropolitan Police commissioner, said during a news conference Friday.

Sky News, citing Reuters, was reporting that a man was also arrested at the Snow Hill rail station in Birmingham and two suitcases are "being dealt with." It's believed the two suitcases could contain explosives.

Since Thursday's attempted bombings — which left only one person injured because the bombs didn't detonate properly — police have been analyzing surveillance tapes, taking statements and conducting house-to-house inquiries. Calling the investigations and tips "fruitful," Hayman appealed to anyone with information about the identity or whereabouts of the four suspects to call police but to not approach the men.

The photos, taken from closed-circuit TV cameras, showed one man wearing a dark shirt with "New York" or "New You" across the front running through a subway station. Another was shown on the upper level of a double-decker bus, while the other two men were shown at separate subway stations.

Earlier in the day, police shot and killed a man believed to be one of the people responsible for Thursday's bombings.

During the press conference, Metropolitan Police Commissioner Ian Blair (search) confirmed that the "shooting is directly linked to the ongoing and expanding anti-terrorist operation ... I need to make clear that any death is deeply regrettable."

But later in the day, sources at Scotland Yard told Sky News reporter Martin Brunt that the man shot was not tied to the actual bombings. He is, however, being described as a man who emerged as being "of interest" in the investigation and is also described as a "terrorist suspect."

"There's a lot of police activity going on today in relation to the Underground and bus incidents yesterday. It's a very, very fast moving investigation," Blair continued. "This is the greatest operational challenge ever faced by the Metropolitan Police Services ... [police are facing] previously unknown threats and great danger."

Blair and Hayman called on all residents and visitors in all communities to help and cooperate with the investigation.

"We remain resolute and will work tireless and imaginatively with every community in London to solve this crime," Blair said. "This operation is targeted against criminals, it is not targeted against any community or any section of the community."

Meanwhile, British officers were at an address in the area of West Kilburn in London in connection with the ongoing investigation into Thursday's bombings, according to a statement from Scotland Yard. As a precaution, armed officers were in attendance and detectives were searching the premises. No arrests have been made.

There are two other addresses being searched, British police officials said during the news conference. Police officials stressed that all situations were under control.

Man Shot Was Not Terror Suspect

The man shot earlier in the day, described by passengers as South Asian, was wearing a thick coat when he ran into the Stockwell subway station on Friday. Temperatures in London on Friday were in the 70s. Police began following the man when he left his home in an effort to arrest him as part of their investigation.

"[The suspect] was challenged and refused to obey police instructions," Blair said.

Witnesses said about 10 armed, plain-clothed police officers chased the suspect. He tripped, and, after telling bystanders to get down, police shot him dead.

"They pushed him onto the floor and unloaded five shots into him. He's dead," witness Mark Whitby told the British Broadcasting Corp. "He looked like a cornered fox. He looked petrified."

Whitby said the man didn't appear to have been carrying anything but said he was wearing a thick coat that looked padded.

Another witness, Anthony Larkin, told the BBC that the man appeared to have "a bomb belt and wires coming out." Sky News reported that no explosives were found on the man after he was shot.

British police do not normally carry firearms, but it is believed that they were given orders to shoot to kill if they believe someone is about to detonate a bomb.

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Service on the Northern and Victoria Tube lines, which pass through Stockwell, was suspended because of the shooting, British Transport Police said. Stockwell is one station away from the Oval station, which was affected by Thursday's attacks.

Scotland Yard said police were investigating a suspect package at Vauxhall, the next station along the Victoria line from Stockwell.

In the West Kilburn incident that was still ongoing, witnesses told Sky News that two women and a boy were searched at a Thai restaurant on Harrow Road.

Sky News' Brunt described the happenings as a "raid" connected to the bombing investigation; there were reports of a bomb disposal unit and bomb-sniffing dogs at the scene.

The Sky News reporter said a 200- to 300-yard stretch of Harrow Road has been cordoned off. Eyewitness said police activity centered on the building next door to the Thai restaurant. She saw two women and a teenage boy spread-eagled against the wall and searched and bags of evidence were taken away.

Some sort of armored vehicle was involved, perhaps a remote vehicle. A convoy of police vehicles was lined up on both sides of Harrow Road. A number of police officers, some armed, were standing outside the restaurant.

Elsewhere, police evacuated one of London's largest mosques after a bomb threat before Friday afternoon prayers.

"Someone phoned our director and said there was a bomb inside," said Mohammed Abdul Bari, chairman of the East London Mosque (search).

The Metropolitan Police lifted the cordon about an hour later, saying no armed officers were involved, and the incident appeared unrelated to the subway shooting.

More than 6,000 people were expected for Friday afternoon prayers but there were only about a dozen people inside at the time the threat was telephoned in.

President Bush said Friday the British people "must understand how strongly America stands with them during these trying times."

Investigation Continues

Investigators, meanwhile, searched for fingerprints, DNA and other forensic evidence connected to Thursday's attacks on three subway trains and a double-decker bus, which were hauntingly reminiscent of homicide bombings only two weeks before.

The devices in Thursday's attacks were either small or faulty, and authorities said the only person who needed medical attention was a person suffering an asthma attack. The July 7 bombings on three Underground trains and a bus killed 56 people, including the four bombers.

A statement posted Friday on an Islamic Web site in the name of an Al Qaeda-linked group claimed responsibility for Thursday's attacks. The group, Abu Hafs al Masri Brigade (search), also claimed responsibility for the July 7 bombings. The statement's authenticity could not immediately be verified.

The attacks targeted trains near the Oval, Warren Street and Shepherd's Bush stations. The double-decker bus had its windows blown out on Hackney Road in east London.

Jittery commuters already facing cutbacks in service from the last attack faced more Underground closures Friday.

"People are worried, but if it's going to happen, it's going to happen, isn't it?" said Chidi O'Hanekwu, 23.

Still, he said he found himself being "a bit more paranoid" on the ride in.

Mia Clarkson, 24, defiantly said she refused to change her schedule.

"You've got to keep living, don't you?" she said as she exited the Chancery Lane station after a trip from across town.

Newspapers reflected the city's volatile mood — part defiance, part anxiety.

"Britain will not be beaten," vowed a front-page headline in the Daily Express. "Is this how we must now live?" asked the Daily Mirror over pictures of the attacks' aftermath. The Independent had a similar photomontage and the words: "City of Fear."

Police appealed for witnesses to give information and set up a Web site to receive amateur video of the attacks and their aftermath.

"Clearly, the intention must have been to kill," Blair said. "You don't do this with any other intention."

The London transport agency said the three affected subway stations remained closed Friday, and service was suspended on all or part of several lines. Other lines have been disrupted since the attacks two weeks ago.

Authorities said it was too early to determine whether the attacks were carried out by the same organization as the July 7 blasts or whether they were linked to Al Qaeda.

The Associated Press contributed to this report.