A crowd threw paint bombs, bottles and masonry at police in Belfast early on Friday, injuring eight officers in a further bout of Northern Ireland's sporadic sectarian violence.
A 1998 peace and power-sharing deal largely ended decades of strife in the British province but Belfast remains divided between pro-British Protestants and Catholics who generally continue to favour unification with Ireland.
The violence began on Thursday night and centred around a bonfire in the Divis Street area, a Catholic-dominated part of Belfast, where British police said they came under attack.
"Eight officers were injured, two requiring hospital treatment for head injuries," Northern Ireland police said in a statement. There were eight arrests for public order offences. Police did not specify what triggered the violence.
In another area of Belfast, a man attacked police with a sword and officers fired two plastic bullets in response.
The bonfire was held to mark the anniversary of the 1971 introduction of internment without trial by British authorities during Northern Ireland's so-called "Troubles".
At that time, soldiers swept into Catholic 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.