Dozens of police officers have been injured in Belfast after clashes broke out during protests against a rally marking the anniversary of the introduction of imprisonment without trial in Northern Ireland.
About 26 police officers were injured, five requiring hospital treatment, when they were attacked with missiles by crowds in the city centre, police said.
Police fired plastic bullets and water cannon at the rioters late on Friday during what was the second successive night of trouble in the latest bout of the province's sporadic sectarian violence.
The 1998 Good Friday agreement, which set up a power-sharing government, largely ended the violence in the British-controlled province although occasional attacks and bomb threats continue.
The predominantly Catholic nationalists favour unificaton with the Republic of Ireland while predominantly Protestant loyalists want the province to remain part of the United Kingdom.
Bricks, bottles and fireworks rained down on riot police when they moved in to try to clear loyalist protesters from the city's main thoroughfare - Royal Avenue - to enable a parade by the nationalist side of the community to go past.
"Police have come under heavy and sustained attack by crowds intent on creating disorder," Assistant Chief Constable George Hamilton said, calling on community leaders to work to reduce tensions.
Internment without trial was introduced by British authorities on August 9, 1971. At that time, soldiers swept into nationalist districts and arrested more than 340 people as the British government sought to halt growing Irish Republican Army (IRA) violence aimed at extinguishing rule from London.
Last month the city was hit by several nights of rioting, predominately by loyalist groups.