LONDON, England (AP) — Britain began flooding London’s streets with 16,000 police officers yesterday, 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 UK 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.

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 yesterday, 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 neighbouring city of Salford. Officers were “aware of pockets of minor disorder” and advising people to avoid the centres 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 yesterday. 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 neighbourhood 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 tomorrow, as the political fallout from the rampage takes hold. The crisis is a major test for Cameron’s Conservative-led coalition government.

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.

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.

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.