This sprawling metropolis convulsed in its worst bout of civil unrest in years Sunday as the violence that engulfed a gritty north London neighborhood over the weekend spread to other poor areas after police shot a black resident.

Police were on high alert across the city after rioters set buildings and cars ablaze, looting and clashing with police in the downtrodden north London neighborhood of Tottenham on Saturday night. The violence was spreading to other pockets of the city Sunday, with Scotland Yard deploying mounted forces in the Enfield neighborhood north of Tottenham after looters had smashed shop windows and more than 100 youths occupied the main street, battling with police.

Further reports of unrest came from the southern neighborhood of Brixton, and social networking sites buzzed with suggestions of rising tensions in other parts of the city.

The riots came on the heels of escalating civil protests in Britain, and just as London is gearing up to hold the 2012 Olympic Games. The city is fighting a strain of civil unrest, including violent demonstrations in December in which gangs of protesters vandalized a department store in central London and attacked a Rolls-Royce carrying Prince Charles and his wife, Camilla.

But the ferocity of the Tottenham riots seemed to shock the nation, with police warning Sunday that there were still “isolated pockets of criminality in the Tottenham area.” Television images showed a massive building, cars and a double-decker bus on fire. At least 26 officers and three residents were injured in the riots, as authorities took 55 people into custody.

“These are very distressing scenes for Londoners in general and the local community in particular,” said Stephen Watson, Scotland Yard police commander. “Our intention at this time is to bring things to as swift a conclusion as we can. Our absolute aim is to restore normality.”

The riot stemmed from the killing of Mark Duggan, 29, who was shot Thursday as police attempted to arrest him as part of an investigation of gun violence in London’s black community. Officials struggled Sunday to calm rumors that Duggan had been shot “assassination style” in the head and vowed an independent inquiry.

There were conflicting reports of what turned an initially peaceful protest Saturday into a rampage. Some residents said police had hit a female protester, triggering the violence. But police said the rioting started when demonstrators “attacked” two squad cars, setting them on fire with Molotov cocktails.

The office of Prime Minister David Cameron — who was on vacation in Italy — denounced the violence as “utterly unacceptable.”

David Lammy, a lawmaker and lifelong resident of Tottenham, said the violence was a “disgrace.”

“The community has had the heart ripped out of it,” he said, adding that the fires had left residents homeless and destroyed buildings and public offices. He also seemed to accuse Scotland Yard of not moving swiftly enough to contain the riots as they escalated late Saturday and spread into Sunday.

Lammy called for answers about “what happened on Thursday,” saying Duggan’s shooting had raised “huge questions.” But, he said, “there can be no excuses for the kinds of scenes that I have seen.”

Tottenham, a poor pocket of the city far from central London’s famous landmarks, has a history of troubled relations with police that community leaders and Scotland Yard have strived to overcome in recent decades. In 1985, the community erupted into violence in the Broadwater Farm area after a 49-year-old woman died from a heart attack after police burst into her home.

The latest tensions with police come as Scotland Yard is reeling from major failures in the investigation of Britain’s phone-hacking scandal, which led to the resignation last month of its police chief, Sir Paul Stephenson.