At least eight police officers are in hospital after unrest broke out in Tottenham, north London, when a peaceful protest and vigil turned violent.
A bus, police vehicles and shops were set on fire and missiles including petrol bombs hurled.
There were reports looting going on.
The trouble follows the shooting dead on Thursday of a local father-of-four, Mark Duggan.
He was killed during an exchange of fire when a special police unit detailed to tackle gun crime stopped a taxi he was travelling in to make a planned arrest.
As is standard practice in the UK in such circumstances, an investigation has been launched into exactly what happened.