Police in London arrested dozens of people on Sunday after 26 officers were injured in the city's worst riots in years.
Rioters in Tottenham, north London, torched vehicles and buildings and looted shops in response to the fatal shooting of a local man by police.
Scotland Yard said 42 people were arrested for the violence, which sparked condemnation from Prime Minister David Cameron's office.
"The rioting in Tottenham last night was utterly unacceptable," a Downing Street spokesman said in a statement.
"There is no justification for the aggression the police and the public faced, or for the damage to property. There is now a police investigation into the rioting and we should let that process happen."
Police said they were still having to deal with "isolated pockets of criminality in the Tottenham area involving a small number of people."
The mayhem, which broke out in Tottenham just before sunset on Saturday, followed a protest over the death of a 29-year-old man on Thursday during an apparent exchange of gunfire with police.
The demonstration had been a peaceful rally outside the police station on Tottenham High Road before two police cars were attacked with petrol bombs and set ablaze.
A public double-decker bus was then torched as the violence rapidly spread, with gangs of hooded youths descending on the area.
The situation raged out of control as hundreds ran amok, setting shops and other vehicles on fire.
There was concern that the unrest was fuelled by rapid posts on social media inciting others to join in.
Central London has seen student and trade union protests turn ugly in the last 12 months but this outbreak of rioting is the worst seen for years in the suburbs.
Under a hail of missiles and petrol bombs, riot officers and mounted police battled to regain control of the streets and escort fire crews safely through to tackle the series of blazes.
Rioters kicked in windows as shops were looted, with people pushing away shopping trolleys full of stolen goods.
One eye-witness said the scene resembled the Blitz, or when parts of London burned following German bombing in the Second World War.
"So many people have lost everything. It's just crazy. It looks like it's the the Second World War. It looks like the Blitz where we were living," Tottenham resident Stuart Radose told Sky News television.
"These are very distressing scenes for Londoners," police commander Stephen Watson said.
"It's important we emphasise that the safety of the public is of paramount importance to us ... Our absolute aim is to restore normality." Watson added that police had not anticipated the level of violence.
Tottenham is an ethnically-diverse urban area best known for its English Premier League football club Tottenham Hotspur.
The unrest followed a peaceful march in protest over the death of minicab passenger Mark Duggan, a father-of-four. He died at the scene.
The march began at Broadwater Farm, a 1960s public housing estate in Tottenham that is notorious across Britain.
In 1985, Police Constable Keith Blakelock was hacked to death on the estate in some of the worst urban rioting in Britain in the past 30 years.
David Lammy, the member of parliament for Tottenham, appealed for calm.
"Those who remember the destructive conflicts of the past will be determined not to go back to them," he said Sunday.
"We already have one grieving family in our community and further violence will not heal that pain. True justice can only follow a thorough investigation of the facts."