The raids started shortly after dawn, as scores of police officers fanned out across this industrial city and surrounded the working-class homes of young British men of Pakistani descent. The men had been conspicuously missing since Thursday, the day more than 50 people were killed in bombings in the heart of London in the worst terrorist attack in British history.

When police barged into and searched six houses a few hours later, it became clear that the investigative trail from London had suddenly led to Leeds, an ethnically mixed city of about 700,000 people about 200 miles north of the capital.

British law enforcement officials said three of the four alleged bombers lived here, a low-profile group that had apparently never attracted the attention of police or intelligence officials for involvement in serious crime or radical causes. While authorities did not identify the men or give any details about their backgrounds, neighbors and local leaders said they were British citizens ranging in age from 18 to 35 who did not stand out, and who were known chiefly for their love of cricket and girls.

Neighbors named one of the purported bombers as Shahzad Tanweer, 22, the son of the owner of a local fish-and-chips shop. British newspapers identified the other suspected bombers as Hasib Hussain, 18, a close friend of Tanweer; Rashid Facha, a British-born Pakistani in his twenties; and Jacksey Fiaz, about 35.

One friend of Tanweer, Azi Mohammed, told the Guardian newspaper that the two had played cricket only 10 days ago. "The idea that he was involved in terrorism or extremism is ridiculous," Mohammed told the Guardian. "The idea that he went down to London and exploded a bomb is unbelievable."

Few details emerged that could explain the relationship between the young men from Leeds. And much less was known about what might have led them to blow up their fellow citizens in apparent suicide attacks.

"We've all been horrified by the terrorist attacks in London, and we're even more horrified that the main suspects may be from our area," said Zunaid Karin, a cleric at the Leeds Grand Mosque, the BBC reported.

Karin said people in Leeds were stunned by the developments and worried that the arrests would heighten tensions that have existed in the city since last week's bombings. Speaking about the city's Muslim community, he said, "We've all received verbal and other threats of harm, unfortunately."

The top law enforcement official in the region, West Yorkshire's Chief Constable Colin Cramphorn, echoed the concern. He told residents to stay alert as investigators searched for other people who may have been involved in the conspiracy, but spent more time at a news briefing pleading for civic calm. "I urge people to reject any extreme responses to today's events," he said.

Law enforcement officials said they arrested one man in Leeds on Thursday in connection with the bombing investigation. They said he was taken to London for questioning but did not identify him or say why he was detained.

Muhammed Iqbal, a town council member, told the Associated Press that all of the homes raided in Leeds belong to "British citizens of Pakistani origin."

"This is not good for Muslims," he said. "We have businesses here. There will be a backlash."

Police evacuated about 600 people from the Hyde Park area of Leeds in the late morning after they found explosives during the search of a property. The material was destroyed by bomb technicians in a controlled explosion. Authorities prevented about half of those evacuated from returning to their homes because of fears that more explosives might be hidden nearby.

Many of those evacuated spent the night in shelters. Other residents near the raided houses spent much of the day milling around police barricades, trying to make sense of what was going on.

In the Beeston section of the city, Jodie Reynolds, 40, who lives a block away from one of the suspected bombers, told the Yorkshire Post that it was hard to believe the perpetrators could have come from her neighborhood. "They are selfish, inconsiderate cowards," she said.