Six months after Britain was struck by five nights of rioting, London police launched a major crackdown on streets gangs which politicians blamed for the country's worst disorder for decades.

London's new police chief, installed in the job in the wake of last August's riots, said a new police unit was carrying out raids on Wednesday on some of the 250 gangs accused of carrying out much of the city's most serious and violent crime.

This is a step change in how we tackle gang crime in London, said Metropolitan Police (Met) Commissioner Bernard Hogan-Howe. It will allow us to identify and relentlessly pursue the most harmful gangs and gang members.

Last August, the shooting by armed police of a man in the deprived area of Tottenham, northeast London, sparked rioting and looting which spread across the city and to other urban areas in the country, leaving five dead and scores injured.

The violence ended only when police flooded the streets with officers. Prime Minister David Cameron and other ministers blamed criminality and the growth of youth gangs, although others said inequality and the government's austerity measures and massive public spending cuts were also responsible.

While official reports identified that only about one in 10 of those arrested were members of street gangs, the issue has become a focal point for action by the authorities.

Police say London has about 250 active criminal gangs with some 4,800 members, with about 62 of these considered a major problem. The gangs are blamed for 22 percent of all serious violence, 17 percent of robberies and 14 percent of rapes.

TASK FORCES

The Met has set up the Trident Gang Crime Command, an expansion of a unit that dealt with shootings and gun murders in the city's black community, which can now call upon 1,000 officers with dedicated task forces in areas across the capital.

It will also build on advice from Bill Bratton, the former police chief of New York and Los Angeles who oversaw dramatic falls in crime and gang violence in the two cities, and who was enlisted by Cameron for help after the riots.

As well as targeting the hardcore gangs, the task forces will seek to spot young people of the periphery of gangs and work to divert them away from crime.

We want to prevent young people getting involved in gang offending so we and other agencies are offering ways out to support young people, Hogan-Howe said. Those who refuse our offer of help will be pursued and brought to justice.

The launch comes a day after a community-led inquiry concluded a breakdown of relations between the police and public, record youth unemployment and poverty led to the outbreak of violence in Tottenham.

Previous parliamentary reports have also blamed the police for their slow response to the riots, giving the impression they had lost control for the streets.