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British police struggled to quell riots in London's poorest neighbourhoods for a third consecutive night, as violent groups of masked youths ransacked stores and torched buildings and cars.

By late Monday night, copycat riots had erupted in Liverpool, Birmingham and Bristol, with police making dozens of arrests.

In Hackney, east London, groups of youths struck police in riot gear with fireworks, bottles and pieces of wood Monday evening. In Peckham, south London, a building and an empty bus were set ablaze and lines of cars were torched in nearby Lewisham.

Meanwhile in the west end of the city, rioters set fire to an enormous Sony distribution centre in Ealing. Similar scenes of arson, looting and clashes with police were reported in Camden and Chalk Farm, in north London.

Monday's intense rioting unfolded in spite of police reinforcements, which CTV's London Bureau Chief Tom Kennedy said were brought in earlier Monday in the hopes of averting a third round of violence.

However, hours later, fires were burning and looting was taking place in at least 25 locations across London, according to the Globe and Mail's European affairs columnist, Doug Saunders.

For local authorities, the escalating riots Monday night represented "probably the toughest policing challenge they have faced in a generation," Saunders told Â鶹´«Ã½ Channel from London.

The chaos and destruction was severe enough that phone carrier BT said the local emergency number, 999, was under "major pressure" and pleaded with customers not to use it except in the case of an "absolute emergency."

The unrest also spread outside of the British capital. In the central city of Birmingham, groups of youths in hoods, their faces covered, squared off against riot police, looted and smashed windows of several stores in the downtown area.

In the northern city of Liverpool, police said they were responding to a "number of isolated outbreaks of disorder," and had made dozens of arrests.

The first riots took place Saturday evening at what was supposed to have been a small, peaceful vigil for a man who was shot and killed by police last week in the gritty North-London neighbourhood of Tottenham.

By Sunday night the unrest had spread to several neighbourhoods including Brixton, which was the scene of riots in the 1980s and 1990s. There, groups of youths smashed windows, attacked a police car, set fires and stole videogames and other goods from local shops.

British Home Secretary Theresa May said Monday 225 people have been arrested in connection with the riots. Thirty-five officers had been injured by early Monday, London police said.

Prime Minister David Cameron was on vacation Monday, but his office announced that he would fly back from Italy overnight and would lead a meeting of the government's crisis committee on Tuesday.

Across the British capital, the destruction has been "really quite serious," Kennedy said Monday afternoon.

"There were a lot of buildings set on fire, many cars were burned out and there was particularly a great deal of looting," he said, adding that all the affected nighbourhoods have been plagued by high youth unemployment and "racial difficulties."

Tottenham's main shopping street remained sealed off by police ahead of nightfall on Monday, as steam continued to rise from burned-out buildings.

Residents in the area wondered what led to the rioting, with many speculating that greedy youths had hijacked what was planned to be a peaceful demonstration Saturday.

"It's nothing to do with the man who was shot, is it?" 37-year-old Marcia Simmons told The Associated Press. "A lot of youths ... heard there was a protest and joined in. Others used it as an opportunity to kit themselves out, didn't they, with shoes and T-shirts and everything."

The man whose death preceded the riots, Mark Duggan, has been described as a 29-year-old father of four. Kennedy said Duggan had been under surveillance by police, under the suspicion that he was involved in gun crime.

Few details have been released about the man's death, leading to speculation about the circumstances surrounding his shooting. The Independent Police Complaints Commission is investigating the incident.

There have been signs of rising social tensions across Britain, as the government moves to cut a swollen deficit by removing $130 billion from public spending by 2015. But police have insisted the unrest is due to criminal activity, not to rising social tensions or a lack of security ahead of the 2012 Olympic Games, which the city is due to host.

The youngest person charged in connection with the rioting to date is an 11-year-old boy who is accused of burglary. Police said about 100 of those arrested are under the age of 22.

On Monday, Police Deputy Assistant Commissioner Steve Kavanagh said the unrest had "changed from a local issue into organized criminality." He pledged that a "momentous investigation" would take place to identify the culprits.

Many of the looters appeared to operate in small groups, each person with their faces covered, who used social media to coordinate their activity and elude police.

The city's mayor, Boris Johnson, is on vacation. He has condemned the riots and is due to return to London on Tuesday.

"People have lost their homes, businesses and livelihoods through mindless violence," Johnson said in a statement.

Meanwhile Tottenham resident Marcia Simmons wondered how her neighbourhood would recover.

"We are the ones who have to live here now," said Simmons. "My son was terrified. He slept in my bed."

"I'm thinking it will be like a ghost town now," she added, referring to the area's main thoroughfare. "Why would anyone want to open a business here?"

With files from The Associated Press