Riot police faced off with youths in London on Monday, television pictures showed, in the third day of disorder after some of the worst rioting in the British capital in years took place at the weekend.

Scores of young people gathered in a main street in the inner-city district of Hackney, smashing up buildings and breaking into a truck that became stranded in the middle of the road, pulling out its contents, the pictures showed.

It began when police tried to carry out a stop and search operation, the BBC said, but a police spokesman could not immediately confirm the report.

Riots broke out in the north London district of Tottenham on Saturday night, following a protest against the death of a local man in a police shooting last week, and the violence spread to other parts of the city on Sunday.

The worst-hit districts - Tottenham, Brixton in south London, and now Hackney - are multi-ethnic areas which have high rates of unemployment.

Police said on Monday that they have arrested 100 people so far, condemning the violence as 'copycat' disorder following the weekend unrest.

The riots started after the killing of 29-year-old Mark Duggan on Thursday.

The father-of-four was shot in a taxi in what was initially said to have been an exchange of gunfire. But media reports said ballistics tests now appear to show that police officers were not under attack when they opened fire.

A protest against Duggan's death on Saturday escalated into a riot, with homes torched and two police cars and a double-decker bus set ablaze in the worst such violence in London for years, less than 12 months before it hosts the Olympics.

Police had braced themselves for 'copycat criminal activity' on Sunday, and violence broke out in the southern district of Brixton, in Enfield, Walthamstow and Islington in the north, and on Oxford Street in the city centre.

At least nine police officers were injured, including three who were taken to hospital after being hit by a speeding car. At least 26 officers were hurt on Saturday.

Deputy Prime Minister Nick Clegg on Monday condemned the violence, saying there was "no excuse whatsoever" for the attacks on homes and businesses.

"Let's be clear, the violence we saw last night had absolutely nothing to do with the death of Mr Duggan. It was needless, opportunist theft and violence - nothing more and nothing less," he said.

Police had deployed extra officers in flashpoint areas on Sunday, but there was still widespread looting, with young men seen pushing trolleys laden with electrical goods out of ransacked stores.

In Brixton, which like Tottenham has a long history of tensions with the police, hundreds of people raided an electrical superstore, a Foot Locker sports goods store was set alight, and several shops had their front windows smashed.

Surveying the streets strewn with rocks and store merchandise on Monday morning, Marilyn Moseley, a 49-year-old Brixton resident, said it was 'pathetic'.

"It's just an excuse for the young ones to come and rob shops," she said, adding: "Anger isn't the word - it's pointless."

Fuelled by messages shared on social networking sites such as Twitter, youths also looted shops in Enfield, a north London suburb three miles (five kilometres) from Tottenham, and nearby Walthamstow.

About 50 youths also gathered in Oxford Circus, on the world-renowned Oxford Street shopping artery, causing some damage before police intervened.

"Officers responding to sporadic disorder in a number of boroughs made more than 100 arrests throughout last night and early this morning," Scotland Yard said on Monday, after 61 people were arrested on Saturday night.

A statement added that officers were 'shocked at the outrageous level of violence directed against them'.

The Tottenham riots erupted after a march in protest against Duggan's death on Thursday, during an operation against gun crime in the black community.

Newspaper reports on Monday suggested that tests conducted on a bullet found lodged in a police officer's radio after the shooting came from a police weapon.

Police have said Duggan's death was 'absolutely regrettable' and the Independent Police Complaints Commission (IPCC) has launched an investigation.

But the shooting caused intense anger in Tottenham, one of the capital's most deprived areas.

Tottenham has a history of tensions with police, and was the scene of the brutal murder of police constable Keith Blakelock, who was hacked to death during riots on the Broadwater Farm housing estate in 1985.

Prime Minister David Cameron's spokesman said the violence was 'completely unacceptable', and Home Secretary Theresa May cut short her holiday to return to Britain to discuss the riots with police chiefs.