At least 40 police officers have been injured trying to calm riots in the Northern Irish capital, Belfast.
Trouble erupted as officers arrested six people in connection with a feud among Protestant paramilitaries in which three men have been murdered.
Last week London cautiously welcomed a move by Republican guerrillas to end an armed struggle for independence in Northern Ireland – raising the hopes for peace in the province.
But while sectarian violence has considerably diminished since the first IRA ceasefire in 1997, sporadic outbreaks within the various armed groups on both sides continue.
Among pro-British Protestants or “Loyalists”, 20 people have been killed since 2000.
Police were forced to close off Crumlin Road last night to try to control the riots in which three cars and a lorry were hijacked and set on fire.
Missiles including petrol bombs and paint pots were hurled at officers.