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."
In other developments:
- David Cameron is chairing the government's emergency committee Cobra to discuss the riots and will also meet Home Secretary Theresa May and Metropolitan Police Acting Commissioner Tim Godwin
- Tube stations in the capital that were closed following the riots have now reopened
- The Tramlink service between East Croydon and Wandle Park has been suspended as a result of the fire at Reeves Corner
- Elsewhere, 100 people have been arrested in Birmingham after scores of youths rampaged through the shopping area, smashing windows and looting from shops
- West Midlands Police said a police station in Holyhead Road in Handsworth, Birmingham, was set on fire
- There were reports of cars being damaged in Manchester and of up to 200 youths with masks roaming through Toxteth in Liverpool
- Police in Bristol said they were dealing with outbreaks of disorder involving about 150 people
- Nottinghamshire Police said a police station was attacked in the St Ann's area and 200 tyres were set alight in the street
- 'Isolated' trouble after man shot were dealt with by police in the Chapeltown area of Leeds overnight
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 16:20 BST, 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.
- Several fires broke out in Croydon, including one at a large sofa factory which spread to neighbouring buildings and tram lines
- Police found a 26-year-old man in a car in Croydon suffering from gunshot wounds. He is in a serious condition in hospital
- In Hackney 200 riot officers with dogs and mounted police were located around Mare Street where police cars were damaged
- The area's MP, Diane Abbott, said a London-wide curfew should be considered
- Looters raided a Debenhams store and a row of shops in Lavender Hill in Clapham, as well as shops in Stratford High Street
- Police used armoured vehicles to push back more than 150 people in the Lavender Hill area
- A Sony warehouse in Solar Way, Enfield, a shopping centre in Woolwich New Road, a timber yard in Plashet Grove, East Ham and a building on Lavender Hill were all on fire
- More than 100 people looted a Tesco store in Bethnal Green, the Met said, and two officers were injured
- Cars were set on fire in Lewisham
- A bus and shop were set alight in Peckham
- Buses were diverted as the violence spread to Bromley High Street
- There were reports of looting of phone shops in Woolwich High Street, in south-east London, and set a police car on fire
- Shops and restaurants were damaged in Ealing, west London, and there was a fire in Haven Green park opposite Ealing Broadway Tube
- Carling Cup matches at Charlton, West Ham, Crystal Palace and Bristol City, which were due to be played on Tuesday, have all been postponed at the request of the police
- A friendly between England and Holland at Wembley on Wednesday was also called off
- At Clapham Junction looters stole masks from a fancy dress store to hide their identity
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."
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.
"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.