The riots raging across London have spread to three major British cities.

Police called in reinforcements and armored vehicles in Birmingham, Bristol and Liverpool, where angry mobs vandalized streets and broken into buildings.

The violent protests started this weekend over a fatal police shooting. But some involved in the riots say they've spread because of broader anger over government cuts to public aid.

In all the violence, one police officer was hit by a car and hospitalized. Three people were arrested and accused of trying to kill the officer.

This is the most serious unrest the United Kingdom has seen since race riots set the capital ablaze in the 1980s.

Some 525 arrests have been made.

British Prime Minister David Cameron recalled Parliament from its summer recess Tuesday and tripled the number of police on the streets of London to deal with the rioting.

Cameron said 16,000 officers would be on the streets of the capital Tuesday night - almost tripling the number on the streets Monday night.

"People should be in no doubt that we will do everything necessary to restore order to Britain's streets and to make them safe for the law-abiding," Cameron told reporters after a crisis meeting at his Downing Street office.