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 crowds of poor, diverse and brazen 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 today there would be 16,000 -- almost triple the number present Monday.
Meanwhile, hundreds of people gathered in Twitter-organized crews to sweep up broken glass, clean vandalized buildings and show the world that their city is about more than mindless destruction.
Britain's riots began Saturday when an initially peaceful protest over a fatal 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.
While the rioters have run off with sneakers, bikes, electronics and leather goods, they also have torched stores. 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 for social benefits as an underlying cause for the outburst of violence.
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.
Early today, firefighters were tackling a major blaze at the site of a recycling center and fuel depot in Tottenham, but it was unclear whether the fire was linked to any new outbreak of rioting. Outside 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 closed library in nearby Salford. Looters targeted stores selling designer clothes and expensive consumer electronics.
In the central city of Nottingham, police said rioters hurled firebombs through the window of one police station and set a vehicle on fire outside a second. Eight men were arrested.
In London, stores, offices and nursery schools closed early amid fears of fresh rioting. Cafes, restaurants and pubs also shut down for the night.
Some London residents prepared to defend their homes and stores.
Police offered advice on what actions people could legally take to defend homes 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," police said.
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.
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.
He 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," he said after a crisis meeting at his Downing Street office.
So far 685 people have been arrested in London and 111 charged -- including an 11-year-old boy. The capital's prison cells were overflowing.