Prime Minister David Cameron issued a stern warning that order would be restored by whatever means necessary.
Deputy Assistant Commissioner Stephen Kavanagh said the raids to round up suspects began overnight, and more than 100 warrants would be executed. Police have already arrested almost 900 people in London since trouble began on Saturday, and 371 have been charged.
Hugh Orde, head of the Association of Chief Police Officers, said there would be "hundreds more people in custody" by the end of the day.
Tensions remained high even in the absence of any major incidents, and Cameron has recalled Parliament from its summer recess for an emergency debate on the riots later in the day. He will face questions about what caused the riots, and pressure to reconsider planned police budget cuts, which critics claim will strain an already overstretched force.
Deputy Prime Minister Nick Clegg said the "sociological debate" about the origins of the violence was for the future.
"Right now it's important that people are reassured that their streets are made safe, their homes are made safe and society is allowed to move on," Clegg told BBC radio.
Calm prevailed in London overnight, with a highly visible police presence watching over the capital. The London police said it would keep up the huge operation — involving 16,000 officers — for at least one more night.
There was a brief outbreak of trouble in Eltham, southeast London, where a group of largely white and middle-aged men who claimed to be defending their neighborhood pelted police with rocks and bottles. Police said the incident had been "dealt with" and a group was dispersed.
There were chaotic scenes at courthouses, several of which sat through the night to process scores of alleged looters and vandals, including an 11-year-old boy.
The defendants included Natasha Reid, a 24-year-old university graduate who admitted stealing a TV from a looted electronics store in north London. Her lawyer said she had turned herself in because she could not sleep because of guilt.
Also due to appear in court were several people charged with using social networking sites like Twitter and Facebook to incite violence.
Other cities where looters had rampaged earlier this week also came through the night largely unscathed, though for the first time minor disturbances were reported in Wales.
Even as Cameron promised Wednesday not to let a "culture of fear" take hold, tensions flared in Birmingham, where a murder probe was opened after three men were killed in a hit-and-run incident as they took to the streets to defend shops from looting.
Chris Sims, chief constable of West Midlands Police, said a man had been arrested on suspicion of murder. Police on Thursday were given more time to question him.
"We needed a fightback and a fightback is under way," Cameron said in a somber televised statement outside his Downing Street office after a meeting of the government's crisis committee. He said "nothing is off the table" — including water cannons, commonly used in Northern Ireland but never deployed in mainland Britain.
Senior police officers, however, indicated they had no plans to use water cannons.
The violence has revived debate about the Conservative-led government's austerity measures, which will slash 80 billion pounds ($130 billion) from public spending by 2015 to reduce the country's swollen budget deficit.
Cameron's government has slashed police budgets as part of the cuts. A report last month said the cuts will mean 16,000 fewer police officers by 2015.
London Mayor Boris Johnson — like Cameron, a Conservative — broke with the government to say such cuts are wrong.
"That case was always pretty frail and it has been substantially weakened," he told BBC radio. "This is not a time to think about making substantial cuts in police numbers."
Scenes of ransacked stores, torched cars and blackened buildings have frightened and outraged Britons just a year before their country is to host next summer's Olympic Games, bringing demands for a tougher response from law enforcement.
Britain's soccer authorities were talking with police to see whether this weekend's season-opening matches of the Premier League could still go ahead in London. A Wednesday match between England and the Netherlands at London's Wembley stadium was canceled.
Britain's riots began Saturday when an initially peaceful protest over a police shooting in London's Tottenham neighborhood turned violent. That clash has morphed into general lawlessness in London and several other cities that police have struggled to halt.
While the rioters have run off with goods every teen wants — new sneakers, bikes, electronics and leather goods — they also have torched stores apparently just to see something burn. They were left virtually unchallenged in several neighborhoods, and when police did arrive they often were able to flee quickly and regroup.
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Paisley Dodds, Meera Selva, David Stringer, Cassandra Vinograd and Raphael Satter contributed to this report.
