Protestant hardliners in Belfast hurled bricks and bottles at police, who responded with rubber bullets and water cannon, as the British-ruled province's annual parades season turned violent.
Pro-British Protestants march every summer in the province, a regular flashpoint for sectarian violence as Catholics, many of whom favour unification with Ireland, see the parades as provocative.
Tens of thousands of Orange Order marchers, wearing orange sashes and waving British flags, paraded at more than a dozen venues across the province to mark Protestant King William III's victory at the Battle of the Boyne in 1690 over the deposed Catholic King James II.
Those who are using the cover of protest to attack the police are massively damaging the cause they support.
During melees that lasted for hours, at least 23 officers and several rioters were injured, police said. In some parts of the city, Protestants and Catholics rained missiles down on each other as well as at police.
Nigel Dodds, a politician representing north Belfast at the London parliament, was hit in the head by a missile and taken to hospital.
"Those who are using the cover of protest to attack the police are massively damaging the cause they support," said Northern Ireland's first minister, Peter Robinson, who heads the devolved government.
"Violence is undermining a just cause and runs totally against the wishes of the Orange Order for protest to be entirely peaceful."
More trouble expected
In north Belfast, police deployed a half-dozen armoured cars to block a road so that the Orange Order could not march past the edge of Ardoyne, a Catholic district that has become the most bitterly contested spot on the city map.
A British-appointed parades commission for the first time this year barred the Orange Order from using the main road beside Ardoyne to return on Friday night to their nearby headquarters.
Leaders of the Orange Order vowed to keep a stream of Protestants marching on the confrontation zone until police caved in and permitted the march to pass.
Police also faced angry crowds around Short Strand, the only Catholic enclave in otherwise Protestant east Belfast.
In a sign that security forces expect the trouble to continue over the weekend, police installed portable toilets and stacked pallets of bottled water for officers manning the armoured-car barricades near Ardoyne. Commanders requested several hundred more police reinforcements from Britain, who will arrive on Saturday.
Since a peace agreement was signed in 1998, violence between Catholics and Protestants - which raged on and off for three decades - has largely ended. But much of Belfast remains divided along religious lines and unrest still flares from time to time.