Hundreds of police reinforcements from Britain were deployed on Belfast's rubble-strewn streets after Protestant riots over a blocked march left 32 officers, a senior politician and at least eight rioters wounded.
Police came under attack on Friday from petrol bombs, fireworks, stones and bottles and responded with water cannon during six hours of street battles.
On Saturday, 400 additional officers were deployed to Northern Ireland as violence continued to flare in the already strained country.
The protests, which have lasted for two nights so far, erupted around the traditional Orange Day parade, held on July 12.
One police officer was engulfed in flames when struck by a petrol bomb, but it was quickly extinguished by colleagues.
The officer remained on duty after being examined by medical personnel, a spokesman said.
Thousands of pro-British Protestants march every summer in the British province, a regular flashpoint for sectarian violence as Catholics, many of whom favour unification with Ireland, see the parades as a provocation.
Violence between the two religious groups still sometimes ignites since a peace deal was signed in 1998, which largely ended three decades of strife. Much of Belfast remains divided along religious and nationalist lines.
The Orange Order organises marches to mark the 1690 victory at the Battle of the Boyne by Protestant Prince William of Orange over Catholic King James of England.
The Order was angered this year when authorities ruled they could not walk along a stretch of road that divides the two communities in the majority Catholic district of Ardoyne, one of the most troubled areas of Belfast.
Police enforced the order by blocking their parade route with seven armoured vehicles.
This decision sparked violence on Friday evening, after tens of thousands of Orange Order marchers, wearing orange sashes and waving British flags, paraded at more than a dozen venues across Northern Ireland.
Orange Order leaders rallied thousands of supporters to the spot of the blockade, where some attacked the vehicles and the lines of heavily armoured officers.
Chief Constable Matt Baggott described Friday's violence as "shocking and disgraceful".
He blamed to Orangemen leaders for inciting the violence and derided their leadership as reckless, saying they had no plan for controlling crowds they had summoned.
An Orangeman and MP in the British Parliament Nigel Dodds had gone to the road block on Friday to appeal for calm but was knocked unconscious by a brick thrown from his own side.