In Britain, violence today raged across London and spread to three other cities. Police clashed with rioters to contain the country's most serious unrest, which has raised questions over the security for the 2012 Olympics. Rioting and looting has spread across London, with unrest flaring up in the central city of Birmingham, the western city of Bristol and the northwestern city of Liverpool. The third cricket Test between India and England is scheduled to be held at Edgbaston, Birmingham, from tomorrow. On the third night of riots in London, gangs of masked youth last night looted shops, attacked police officers and set fire to vehicles and garbage dumps in raging violence, sparked by the killing of a youth in police shooting in Tottenham. Clashes in Hackney between police and groups of youths were shown live through helicopter cameras, besides at least one car set on fire on the third night of violence. Over 200 people have been arrested till last night as trouble spread to newer areas. Scotland Yard has deployed an extra 1,700 officers to deal with the unrest in the capital. Violence in Britain had started on Saturday in Tottenham district after a peaceful protest over the police shooting of Mark Duggan, a 29-year-old local man, turned violent. Trouble has since spread to areas of London, such as Islington, Enfield, Walthamstow, Oxford Circus and now Hackney. Prime Minister David Cameron is returning early from holiday to discuss the unrest, which appeared to be led by youths alienated by years of under employment which left them feeling marginalized even before the economic downturn.