LONDON (AP) -- Britain began flooding London's streets with 16,000 police officers Tuesday, tripling their presence as the nation feared its worst rioting in a generation would stretch into a fourth night. The violence has turned buildings into burnt out carcasses, triggered massive looting and spread to other U.K. cities.

Police said they were working full-tilt, but found themselves under attack - from rioters roaming the streets, from a scared and worried public, and from politicians whose cost-cutting is squeezing police numbers ahead of next year's Olympic Games.

London's Metropolitan Police force vowed an unprecedented operation to stop more rioting, flooding the streets Tuesday with 16,000 officers over the next 24 hours, nearly three times Monday's total.

Although the riots started Saturday with a protest over a police shooting, they have morphed into a general lawlessness that police have struggled to halt with ordinary tactics. Police in Britain generally avoid tear gas, water cannons or other strong-arm riot measures. Many shops targeted by looters had goods that youths would want anyway - sneakers, bikes, electronics, leather goods - while other buildings were torched apparently just for the fun of seeing something burn.

Police said plastic bullets were "one of the tactics" being considered to stop the looting. The bullets were common in Northern Ireland during its years of unrest but have never before been used in mainland Britain.

But police acknowledged they could not guarantee there would be no more violence. Stores, offices and nursery schools in several parts of London closed early amid fears of fresh rioting Tuesday night, though pubs and restaurants were open. Police in one London district, Islington, advised people not to be out on the streets "unless absolutely necessary."

"We have lots of information to suggest that there may be similar disturbances tonight," Commander Simon Foy told the BBC. "That's exactly the reason why the Met (police force) has chosen to now actually really 'up the game' and put a significant number of officers on the streets."

The riots and looting caused heartache for Londoners whose businesses and homes were torched or looted, 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 criminology at the University of East London.

London's beleaguered police force called the violence the worst in decades, noting they 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.

Police said there had been "some disorder" Tuesday afternoon in towns near Birmingham, with shops broken into in Wolverhampton and two cars set alight in West Bromwich.

Manchester police Assistant Chief Constable Terry Sweeney said his department was handling small outbreaks of unrest in Manchester and the neighboring city of Salford. Officers were "aware of pockets of minor disorder" and advising people to avoid the centers of both areas.

Police launched a murder inquiry after a man found with a gunshot wound during riots in the south London suburb of Croydon died of his injuries Tuesday. Police said 111 officers and 14 members of the public were hurt over the three days of rioting, including a man in his 60s with life-threatening injuries.

So far more than 560 people have been arrested in London and more than 100 charged, and the capital's prison cells were overflowing. Several dozen more were arrested in other cities.

The Crown Prosecution Service said it had teams of lawyers working 24 hours a day to help police decide whether to charge suspects.

Prime Minister David Cameron - who cut short a holiday in Italy to deal with the crisis - recalled Parliament from its summer recess for an emergency debate on the riots and looting that have spread from the deprived London neighborhood of Tottenham to districts across the capital, and the cities of Birmingham, Liverpool and Bristol.

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 police 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.

Parliament will return to duty on Thursday, as the political fallout from the rampage takes hold. The crisis is a major test for Cameron's Conservative-led coalition government.

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.

A wave of violence and looting raged across London on Monday night, as authorities struggled to contain the country's worst unrest since race riots set the capital ablaze in the 1980s. Groups of young people rampaged for a third straight night, setting buildings, vehicles and garbage dumps alight, looting stores and pelting police officers with bottles and fireworks.

Rioters, able to move quickly and regroup to avoid the police, were left virtually unchallenged in several neighborhoods, plundering stores at will.

Silke said until police were seen arresting large numbers of rioters, it will be hard to control the rioting.

"People are seeing images of lines of police literally running away from rioters," he said. "For young people that is incredibly empowering. They are breaking the rules, they are getting away with it, no one is able to stop them."

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!" during a walkabout 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 "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 first 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 - a revelation that could fuel the anger of the local community.

An inquest into Duggan's death was opened Tuesday, though it will likely be several months before a full hearing.

Duggan's death stirred memories of the bad old days 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.

Others pointed to rising social tensions in Britain as the government slashes 80 billion pounds ($130 billion) from public spending by 2015 to reduce the huge deficit, swollen after the country spent billions bailing out its foundering banks.

Many rioters appeared to relish the opportunity for violence. "Come join the fun!" shouted one youth as looters hit the east London suburb of Hackney.

In Hackney, one of the boroughs hosting next year's Olympics, hundreds of youths left a trail of burning trash and shattered glass. Looters ransacked a convenience store, filling plastic shopping bags with alcohol, cigarettes, candy and toilet paper.

Disorder flared throughout the night, from gritty suburbs along the capital's fringes to west London's posh Notting Hill neighborhood.

In Croydon, fire gutted a 140-year-old family run department store, House of Reeves, and forced nearby homes to be evacuated.

"I'm the fifth generation to run this place," said owner Graham Reeves, 52. "I have two daughters. They would have been he sixth.

"No one's stolen anything," he said. "They just burnt it down."

In the Clapham Junction area of south London, a mob stole masks from a party store to disguise their identities and then set the building on fire. In nearby Peckham, a building and a bus were set ablaze. Cars were torched in nearby Lewisham, and in west London's Ealing suburb the windows of each store along entire streets had been smashed.

A blaze gutted a Sony Corp. distribution center in north London, damaging DVDs and other products, and about 100 young people clashed with police in north London around Camden.

"We locked all the doors, and my wife even packed a bag to flee," said 27-year-old Camden resident Simon Dance. "We had Twitter rolling until midnight just to keep up with the news. We were too afraid to even look out the window."

Outside London, dozens of people attacked shops in Birmingham's main retail district, and clashed with police in Liverpool and Bristol.

On Tuesday, as Londoners emerged with brooms to help sweep the streets of broken glass, many called for police to use water cannons, tear gas or rubber bullets to disperse rioters, or bring out the military for support. Although security forces in Northern Ireland regularly use all those methods, they have not been seen on the mainland in decades.

Conservative lawmaker Patrick Mercer said that policy should be reconsidered.

"They should have the tools available and they should use them if the commander on the ground thinks it's necessary," he said.

But the government rejected the calls, for now.

"The way we police in Britain is not through use of water cannon," Home Secretary Theresa May told Sky News. "The way we police in Britain is through consent of communities."

The riots could not have come at a worse time for police, a year before the Olympic Games, which Scotland Yard says will be the biggest challenge in its 182-year history.

The government has slashed police budgets as part of its spending cuts. A report last month by Her Majesty's Inspectorate of Constabulary said the cuts - a third of which have already taken place - will mean 16,000 fewer police officers by 2015.

Opposition Labour lawmaker David Winnick said the government should scrap its plan to cut police numbers.

"I think it's absolute madness in view of what's happened over the last few nights," he said.

The force also is without a full-time leader after chief Paul Stephenson quit last month amid a scandal over the ties between senior officers and Rupert Murdoch's British newspapers, which are being investigated for hacking phone voicemails and bribing police for information. The force's top counterterrorism officer, John Yates, also quit over the hacking scandal.

Police representatives say officers are demoralized, and feel a sense of betrayal by politicians and their leaders.

Constable Paul Deller, a 25-year veteran working in a police control center during Monday's violence, said the rioting was "horrific."

He acknowledged there were not enough officers on the streets to stop it, but said "we gave it everything we could."

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David Stringer, Raphael Satter, Sheila Norman-Culp, Meera Selva and Stephen Wilson contributed to this report.

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