LONDON - Thousands more police officers flooded London streets Tuesday in a bid to end Britain's worst rioting in a generation as nervous shopkeepers closed early and some residents stood guard to protect their neighborhoods. An eerie calm prevailed in the city, but unrest spread across central and northern England on a fourth night of violence driven by poor, diverse and brazen crowds of young people.
Scenes of ransacked stores, torched cars and blackened buildings frightened and outraged Britons just a year before London is to host the summer Olympic Games, and brought demands for a tougher response from law enforcement.
London's Metropolitan Police department put thousands more officers in the streets and said that by Wednesday there would be 16,000 - almost triple the number present Monday. The department said a large presence would remain in the city through the next 24 hours at least.
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 a general lawlessness in London and several other cities that police have struggled to halt with ordinary tactics.
While the rioters have run off with sneakers, bikes, electronics and leather goods, they also have torched stores apparently just for the fun of seeing something burn. They were left virtually unchallenged in several neighborhoods, and when police did arrive they often were able to flee quickly and regroup.
Some saw Britain's economic crisis and deep cuts planned to socials benefits as a deeper underlying cause for the outburst of violence - though few rioters said it was their motivation.
The show of strength by police appeared to have quelled unrest in London late Tuesday, but in a move that could raise tensions, a far-right group said about 1,000 of its members around the country were taking to the streets to deter rioters.
"We're going to stop the riots - police obviously can't handle it," Stephen Lennon, leader of the far-right English Defense League, told The Associated Press. He warned that he couldn't guarantee there wouldn't be violent clashes with rioting youths.
Anders Behring Breivik, who has confessed to the bombing and massacre that killed 77 people in Norway last month, has cited the EDL as an inspiration.
Firefighters were tackling a major blaze at the site of a recycling center and fuel depot in Tottenham early Wednesday, but it was unclear whether the fire was linked to rioting.
Outside of London, chaos continued to spread.
In the northwestern city of Manchester, hundreds of youths rampaged through the city center, hurling bottles and stones at police and vandalizing stores. A women's clothing store on the city's main shopping street was set ablaze, along with a disused library in nearby Salford. Looters targeted stores selling designer clothes and expensive consumer electronics.
Manchester's assistant chief constable Garry Shewan said looting and arson had taken place there on an unprecedented scale, but appeared to have little motive. About 50 people were arrested there.
"We want to make it absolutely clear - they have nothing to protest against. There is nothing in a sense of injustice and there has been no spark that has led to this," he said.
In the central England city of Nottingham, police said rioters hurled firebombs though the window of a police station, and set fire to a school and a vehicle outside a second police station - but there were no reports of injuries. A total of 90 people were arrested in attacks on stores, a college, a community center and cars.
Neither Manchester nor Nottingham had previously been involved in unrest. There also were minor clashes for the first time in the central England locations of Leicester, Wolverhampton and West Bromwich.
In London, stores, offices and nursery schools closed early amid fears of fresh rioting. Several usually busy streets were quiet as cafes, restaurants and pubs also decided to shut down for the night.
Many shops had their metal blinds pulled down, while other business owners rushed to secure plywood over their windows before nightfall.
Some London residents prepared to defend their homes and stores. Outside a Sikh temple in Southall, west London, residents stood guard and vowed to defend their place of worship if mobs of young rioters appeared. Another group marched through Enfield, in north London, aiming to deter looters.
In east London's Bethnal Green district, convenience store owner Adnan Butt said residents were tense.
"People are all at home - they're scared" of the rioters, he said.
Police offered advice on what actions businesses or homeowners could legally take to defend properties from attack. "As a general rule, the more extreme the circumstances and the fear felt, the more force you can lawfully use in self-defense," London police said in advice circulated late Tuesday.
Senior officers said they were considering the possible use of plastic bullets - blunt-nosed projectiles designed to deal punishing blows to rioters without penetrating the skin. Such weapons, formally called baton rounds, still are used to quell riots in Northern Ireland but have never been used by police on Britain's mainland.
Prime Minister David Cameron's government rejected calls by Conservative lawmaker Patrick Mercer and some members of the public for strong-arm riot measures that British police generally avoid, such as tear gas and water cannons.
"They should have the tools available and they should use them if the commander on the ground thinks it's necessary," Mercer said.
The disorder has caused heartache for Londoners whose businesses and homes were torched or ransacked, and a crisis for police and politicians already staggering from a spluttering economy and a scandal over illegal phone hacking by a tabloid newspaper that has dragged in senior politicians and police.
"The public wanted to see tough action. They wanted to see it sooner and there is a degree of frustration," said Andrew Silke, head of the criminology department at the University of East London.
So far 685 people have been arrested in London and 111 charged - including an 11-year-old boy - and the capital's prison cells were overflowing. Britain's Crown Prosecution Service said it had teams of lawyers working 24 hours a day to help police decide whether to charge suspects, allowing them quickly clear police station cells.
About 230 people were arrested after two days of violence in Birmingham - where police were investigating reports of shots fired in a restive inner-city neighborhood. "Officers are on the scene but there have been no injuries reported," the city's police department said in a statement.
In the northern city of Liverpool, about 200 youths hurled missiles at police and firefighters in a second night of unrest, and about 40 were arrested.
A total of 111 officers and 14 members of the public have been hurt so far in the rioting, including a man in his 60s who was attacked as he attempted to put out a fire started by members of a mob.
Police said the injured man had been tackling a blaze in a garbage bin, when he was set upon by several rioters. "It was quite a grave assault and his condition is causing us some concern," said police commander Simon Foy.
The unrest has been Britain's worst since race riots set London ablaze in the 1980s. London's beleaguered police force noted that it had received more than 20,000 emergency calls on Monday - four times the normal number. Scotland Yard has called in reinforcements from around the country and asked all volunteer special constables to report for duty.
A soccer match scheduled for Wednesday between England and the Netherlands at London's Wembley stadium was canceled to free up police officers for riot duty. Britain's soccer authorities said they were in talks with police to see whether this weekend's season-opening matches of the Premier League could still go ahead in London.
Prime Minister David Cameron cut short a holiday in Italy to deal with the crisis, reversing an earlier decision to remain on his vacation. He recalled Parliament from its summer recess for an emergency debate on the riots Thursday.
Cameron described the scenes of burning buildings and smashed windows as "sickening," but refrained from tougher measures such as calling in the military to help restore order.
"People should be in no doubt that we will do everything necessary to restore order to Britain's streets and to make them safe for the law-abiding," Cameron told reporters after a crisis meeting at his Downing Street office.
Other politicians visited riot sites Tuesday - but for many residents it was too little, too late.
Deputy Prime Minister Nick Clegg was booed by crowds who shouted "Go home!" in Birmingham, while London Mayor Boris Johnson - who flew back overnight from his summer vacation - was heckled on a shattered shopping street in Clapham, south London.
Johnson said the riots would not stop London from "welcoming the world to our city" for the Olympics.
"We have time in the next 12 months to rebuild, to repair the damage that has been done," he said. "I'm not saying it will be done overnight, but this is what we are going to do."
Violence broke out late Saturday in the low-income, multiethnic district of Tottenham in north London, after a protest against the fatal police shooting of Mark Duggan, a 29-year-old father of four who was gunned down in disputed circumstances Thursday.
Police said Duggan was shot dead when officers from Operation Trident - the unit that investigates gun crime in the black community - stopped a cab he was riding in.
The Independent Police Complaints Commission, which is investigating the shooting, said a "non-police firearm" was recovered at the scene, but that there was no evidence it had been fired. An inquest into Duggan's death was opened Tuesday, but a full hearing will likely take several months.
Duggan's death stirred memories of the 1980s, when many black Londoners felt they were disproportionately stopped and searched by police. The frustration erupted in violent riots in 1985.
Relations have improved since then, but tensions remain and many young people of all races mistrust the police.
Seeking explanations for the unrest, some pointed to rising social tensions in Britain as the government slashes 80 billion pounds ($130 billion) from public spending by 2015 to reduce the country's huge budget deficit, swollen after the country spent billions bailing out its foundering banks.
But many rioters appeared simply to relish the opportunity for unchecked violence Monday night. "Come join the fun!" shouted one youth as looters hit the east London suburb of Hackney.
In Croydon, fire gutted a 140-year-old family run department store, House of Reeves, and forced nearby homes to be evacuated. "No one's stolen anything," said owner Graham Reeves, 52. "They just burnt it down."
Police said a 21-year-old man was arrested late Tuesday in connection with the blaze.
Paisley Dodds, Jill Lawless, Danica Kirka, Meera Selva, Sheila Norman-Culp and Stephen Wilson contributed to this report.