More than 300 volunteers took to the streets of London to clean up the remains of vehicles destroyed in riots in the north following three nights of unrest.

About 16,000 police will be out patrolling the streets to prevent a fourth night of unrest, which has spread to Manchester, northern England's biggest city. Rioters in Manchester reportedly set fire to a property in Salford, west of the center, and a clothing retail store in the main shopping area, Greater Manchester Police said.

A total of 685 people have been arrested in London since Aug. 8, when the unrest began in the gritty neighborhood of Tottenham, where a local black man, Mark Duggan, 29, was fatally shot by police who stopped his car with the intention of making an arrest. The violence resulted in looting and the burning of police cars and businesses.