LONDON – Thousands of extra police officers flooded the streets of London on Wednesday to deter rioters, and Prime Minister David Cameron warned that the government will take any necessary action to restore order and confidence to Britain’s streets.

Even as Cameron promised not to let a “culture of fear” take hold, tensions flared in Birmingham, where a murder investigation was opened after three men were killed by a hit-and-run driver as they took to the streets to defend shops from looting.

“We needed a fight-back and a fight-back is under way,” Cameron said in a somber televised statement outside his Downing Street office after a meeting of the nation’s crisis committee. As if to indicate his resolve, he said “nothing is off the table.”

The number of arrests in London climbed to 820, with courts staffing around the clock to process alleged looters, vandals and thieves — including one as young as 11. Cameron has recalled Parliament from its summer recess for an emergency debate on the riots today.

An eerie calm prevailed over most of London as night fell Wednesday, with a highly visible police presence throughout the city. Metropolitan Police said objects had been thrown at officers in south London’s Eltham neighborhood but that the incident had been “dealt with” and a group was dispersed.

In England’s second-largest city of Birmingham, police launched a murder investigation after three men were hit and killed by a car. Residents said the victims, age 21 to 31, were members of Birmingham’s South Asian community who had been patrolling their neighborhood to keep it safe from looters.

“They lost their lives for other people, doing the job of the police,” said witness Mohammed Shakiel, 34. “They weren’t standing outside a mosque, a temple, a synagogue or a church — they were standing outside shops where everybody goes. They were protecting the community.”

Tariq Jahan, whose 21-year-old son Haroon was killed, stood in a Birmingham street and pleaded with the South Asian community not to seek revenge against the car’s occupants, reported to be black.

“Today we stand here to plead with all the youth to remain calm, for our community to stand united,” he said. “This is not a race issue. The family has received messages of sympathy and support from all parts of the community — all races, all faiths and backgrounds.”

He urged angry young men in the streets to “grow up” and go home.

Chris Sims, chief constable of West Midlands Police, said a man had been arrested on suspicion of murder.

“The information we have at the moment would support the idea that the car was deliberately driven,” he said, appealing for calm. “My concern would be that that single incident doesn’t lead to a much wider level of distress and even violence between different communities.”

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 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 the next summer Olympic Games, bringing demands for a tougher response from law enforcement. Police across the country have made almost 1,200 arrests since the violence broke out in the capital on Saturday.

Armored vehicles and convoys of police vans backed up some 16,000 officers on duty. The show of force seems to have worked — there were no reports of major trouble in London on Wednesday night, although police continued to make arrests.

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.