Rioting has spread across London on a third night of violence, with unrest flaring in other English cities.
An extra 1,700 police officers were deployed in London, where shops were looted and buildings were set alight.
Birmingham, Liverpool, Nottingham and Bristol also saw violence.
The prime minister has returned early from his holiday to discuss the unrest, which first flared on Saturday after a peaceful protest in Tottenham over the fatal shooting of a man by police.
At least 400 people have been arrested following a wave of "copycat criminal activity" across London over the past three days, the Met Police said. More than 69 people have been charged with various offences.
Three people are being questioned on suspicion of attempted murder after a police officer was injured by a car in Wembley, north-west London, while trying to stop suspected looters.
Met Deputy Assistant Commissioner Steven Kavanagh said it was a "shocking and appalling morning for London to wake up to".
"The Met was stretched beyond belief in a way that it has never experienced before," he told BBC Breakfast.
Acting Commissioner Tim Godwin ruled out bringing in the Army to help police tackle the violence, but said: "We will be out there in ever greater numbers tonight."
On Twitter, Scotland Yard said: "In the next 24 hours there will be 13,000 police officers on duty in London."
Monday's violence started in Hackney after a man was stopped and searched by police but nothing was found.
Groups of people began attacking the police in Hackney at about 15:20 GMT, throwing stones and a bin at officers.
Police cars were smashed by youths armed with wooden poles and metal bars. Looters also smashed their way into shops before being dispersed by police.
Nine police forces from other parts of the country have assisted in providing support to the capital city, as well as the City of London Police and British Transport Police.
However, eyewitnesses have reported that as trouble spread across the city, there were often few police officers around when violence flared.
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Looters in a convenience store in Hackney Shops have been looted across the capital
Catherine Holmes, a resident in Hackney, said: "The common feeling in Hackney Central is that our community has been hurt and damaged by causeless violence.
"We spoke to looters trying to get home - the only explanation they gave for their behaviour was that they had no money today.
"It is sad to think that these people are thinking of only the next moment, and the moment they have created is a nightmare."
'War zone'
Ealing resident Christian Potts, 29, was driving through the area when he witnessed the disturbances.
"It looks like a war zone - I have never seen anything like it in all my life," he said.
"There were about 25 to 30 masked youths on Haven Green and they just started tearing into a florist with bricks.
Clapham resident Nick Shaw escaped unhurt from his home, which was set ablaze
"It's a local family-run business so I can't see why they are doing this."
London's mayor Boris Johnson is cutting short his holiday to return to the city.
Home Secretary Theresa May also returned early from holiday, to meet Met chiefs to discuss their response to the violence.
"These have been the worst scenes of violence and disturbance on our streets for many, many years, and this sort of violence, this level of criminality, this thuggery, this looting, this theft, is completely unacceptable," Ms May told BBC Breakfast.
"We can deal with it. We can deal with it with robust policing, with good use of intelligence, but also with the help and support of local communities."
She added: "If there's anybody who knows somebody who was out on those streets last night and involved in this action then they should tell the police."
The trouble follows two nights of violence over the weekend, which started after police shot a man dead in Tottenham.
A peaceful protest in Tottenham on Saturday over the death of Mark Duggan, 29, was followed by violence which spread into this week.