A peaceful protest against the killing of the 29-year-old man in north London's Tottenham area degenerated into a Saturday night rampage, with rioters torching a double-decker bus, destroying patrol cars and trashing a shopping mall in the nearby Wood Green district.
Disturbances broke out late Sunday in Enfield, about 5 miles (8 kilometers) north of Tottenham. TV footage showed riot and mounted police patrolling the streets, and there were also images of smashed shop windows, and police with dogs detaining at least one man.
There were also reports that a police car was vandalized in Enfield. Sky News television reported that several hundred young people were on the streets causing trouble.
"We do have extra resources out tonight on duty across the capital," police commander Christine Jones said. "We are carefully monitoring any intelligence and ensuring we have our resources in the right places. No one wants to see a repeat of the scenes that we witnessed last night in Tottenham."
In Saturday's violence, several buildings were set ablaze. TV footage showed the double-decker bus in a fireball and mounted police charging through the streets trying to restore order. Police said 26 officers received injuries, most if not all apparently minor, and made 55 arrests, including four Sunday. The majority of arrests were for burglary; other offenses included violent disorder, robbery, theft and handling of stolen goods.
London's fire department said it dealt with 49 "primary" fires in Tottenham. No firefighters were injured.
Social networking websites swirled with rumors of other riots beginning or being planned in other areas of the city, but police warned the public not to trust everything they saw on the Internet - adding that officers were keeping a close eye on what was being said online as well.
The violence has cast a pall over a city preparing to host the 2012 Olympic Games.
"I hope people will have a fantastic Olympics no matter what happened last night," London Mayor Boris Johnson said in a telephone interview with BBC television, trying to assure the world his city was safe.
Others weren't so sure, suggesting that the riots had exposed incipient tensions at a time of sharp public sector cutbacks and economic uncertainty.
"This is just a glimpse into the abyss," former Metropolitan Police Commander John O'Connor told Sky News. "Someone's pulled the clock back and you can look and see what's beneath the surface. And what with the Olympic Games coming up, this doesn't bode very well for London."
The protest against the death of Mark Duggan, a father of four who was gunned down in disputed circumstances Thursday, was initially peaceful. But it got ugly as between 300 and 500 people gathered around Tottenham's police station. Some protesters filled bottles with gasoline to throw at police lines, others confronted officers with makeshift weapons - including baseball bats and bars - and attempted to storm the station.
Within hours, police in riot gear and on horseback were clashing with hundreds of rioters, fires were raging out of control, and looters combed the area. One video posted to the Guardian newspaper's website showed looting even carried on into the following day, with people even lining up to steal from one store just after dawn.
The devastated area smoldered Sunday - in Tottenham, streets were littered with bricks and lined with overturned scorched trash cans. Two police helicopters hovered over the burnt-out buildings as residents inspected the damage and firefighters doused the last of the flames. Glaziers were busy replacing the smashed windows of looted shops.
Very few details of Duggan's death have been released, although police said initially that an officer was briefly hospitalized after the shooting - suggesting there was some kind of an exchange of fire. Media reports said a bullet had been found lodged in the officer's police radio.
The Independent Police Complaints Commission, which is investigating Duggan's shooting, provided more details in a statement Sunday night, saying a "non-police firearm" was recovered at the scene. It added that the weapon and a police radio "have been sent for forensic testing."
Duggan's family rejected any suggestion that he had fired at officers. His brother, Shaun Hall, said his sibling would never attack police.
"That's ridiculous," he told Sky News television. As for the rioting, he condemned it.
"There was a domino effect, which we don't condone at all," he said.
Local lawmaker David Lammy, speaking to residents from behind police tape earlier in the day, said that Duggan's shooting "raised huge questions and we need answers," but he warned against renewed violence.
"The response to that is not to loot and rob," he said. "This must stop."
Tottenham has a history of unrest. It was the site of the 1985 Broadwater Farm riots, a series of clashes that led to the savaging stabbing of a police officer and the wounding of nearly 60 others - brutally underscoring tensions between London's police and the capital's black community.
Relations have improved since, but mistrust still lingers.
Juergen Baetz, Jill Lawless and Frank Griffiths contributed to this report.