London: As Britain simmered under one of its worst riots that left three Asian-origin men dead, Premier David Cameron on Wednesday said a "fightback is underway" and empowered his forces to use water cannons to crush the violent unrest that spread to central and northern cities from London.

Vowing not to allow "a culture of fear" to take over the streets, Cameron said water cannons -- never used in mainland UK -- would now be made available to police, apart from plastic bullets, to deal with violence after four nights of rioting and looting by hooded youths who ransacked stores and torched vehicles.

His comments came as 16,000 police officers flooded the streets of London, where clam returned, as Asian and other vigilante groups sprouted to protect their stricken neighbourhood from roving gangs, who had also burnt down and raided houses and shopping complexes.

Outside a gurdwara in Southall in west London, Sikhs vowed to protect their place of worship from the rioters.

"The continued violence is unacceptable," Cameron announced after chairing the government's emergency response committee, which took stock of raging violence.

"It will be stopped," the British Prime Minister said as he ridiculed the rioters as "sick".

The police and the law enforcement agencies would not be short of resources and would have the full backing of the state behind them, he said, announcing that a "fightback is underway."

Though a massive police presence ensured that London remained calm now, the copycat riots spread to cities of Manchester, Salford, Liverpool, Wolverhampton, Nottingham, Leicester and Birmingham.

More than 1,000 people have been arrested during four nights of rioting and Cameron said Britain will set up extra courts to quickly prosecute "thugs" who indulged in looting and rioting.

Three young men of Asian-origin -- two brothers Shahzad and Harry Hussain, and their friend Musaver Ali – were knocked down by a speeding car as they were trying to guard their communities in Birmingham. Police said they were treating the deaths as murder.

Cameron said the "appalling" violence that continued overnight had claimed three lives in Birmingham where police were trying to get to the bottom of the incident.

"This continued violence is simply not acceptable, and it will be stopped. We will not allow a culture of fear to exist on our streets. We needed a fightback and a fightback is underway," he told reporters outside 10, Downing Street.

The authorities, Cameron said, would have no hesitation in "naming and shaming" the culprits and rejected that concerns over human rights might hold back law enforcement agencies from publication of photographs of those indulging in hooliganism and violence.

He said his government was determined to bring the full force of law on perpetrators of violence and courts were working overnight for speedy disposal of cases.

In a hard-hitting statement released after the meeting, Cameron said, "There are parts of our society that are not just broken but frankly sick. When we see children of 12 or 13 looting it's clear there are things that are badly wrong in our society."

Gang violence raged in other major cities of Britain after spreading there from London, where it had erupted following the death of 29-year-old Mark Duggan in police firing on Thursday. Rioters broke into stores and ran off with electronic and leather goods, bikes, sports shoes, branded clothes and jewellery with virtually no resistance from the police and security personnel.

In central Nottingham, the Canning Circus police station was firebombed by a gang last evening, while in Liverpool, the Merseyside Police arrested 50 people in relation to disorder in the city.

Cameron said he would make a statement in Parliament tomorrow as it meets for a day-long session to discuss the conflagration that has blighted Britain's image and raised questions over the security for next year's Olympics.

In London where increased police presence brought back calm, a Downing Street spokesperson said the higher police numbers and tough police tactics last night had clearly worked.

"There's a will to continue with that level of policing for as long as is needed."

The strength of police in London was increased from 6,000 to 16,000 with orders to deal with any incidence of rioting and violence in a tough manner. London wore a deserted look last evening with offices and shops closed earlier than usual.

There were indications that the government will reverse its decision to cut police budget that would have meant lower number of officers on the streets of London.

London mayor Boris Johnson said: "This is not a time to think about making substantial cuts in police numbers."

UK drops 'You're invited' tourism ad

With the ongoing riots inviting adverse publicity, Britain's tourism authority has dropped its slick 60-second marketing 'You're invited' video that was put up on BBC and other websites to attract potential tourists from abroad.

A spokesman for VisitBritain said: "We have taken the videos down, they are not appropriate at this time. We have not changed our advertising and marketing strategy, but we have removed those videos for the time being".

The spokesman told The Guardian that the tourism authority was not aware of reports of tourists leaving Britain early or cancelling trips to visit the country due to the riots in London and other parts of Britain.

"At the moment these are isolated incidents occurring away from tourist attractions. Tourists have not been affected," he said.

Mary Rance, head of trade body UKInbound, which represents businesses in the tourism industry, said: "The riots of the past few days, particularly in London, are most unfortunate for the global image of the UK – and not just ahead of the Olympics but for the country's short term and long term inbound tourism industry".

"With scenes of looting, violence and lawlessness flashing across TV screens across the globe it is absolutely vital that the government and its agencies, as well as the UK tourism industry, work hard to put things in context.

"London is still one of the greatest cities in the world with an enviable record of safety. It's important we stress that these developments are not typical". In a statement, the European Tour Operators Association (ETOA) sought to reassure tourists that London remained safe It said: "The events have been largely confined to secondary shopping centres in the suburbs".

"So long as the damage is contained outside of central London, then there will be little long-term impact on demand for London as a tourist destination".

Deborah Griffin, director of Deloitte's leisure corporate finance advisory team, said: "It's certainly not good. All of it has been flashed around the world and that will put people off, but normally people's memories are quite short-term so it should not have a long-term impact".