Police were attacked with bricks and bottles in Belfast during protests against a rally marking the introduction of imprisonment without trial in Northern Ireland.
Four officers were injured, two of them requiring hospital treatment, and two members of the public were hurt in the clashes in the city centre on Friday evening, the Police Service of Northern Ireland (PSNI) said.
A number of parked cars were set on fire and police responded with water cannon and by firing baton rounds.
"Police have come under heavy and sustained attack by crowds intent on creating disorder," Assistant Chief Constable George Hamilton said.
He called on community leaders to work to reduce tensions.
Protesters had tried to block part of the route of a planned republican parade marking the introduction by British authorities of internment without trial on August 9, 1971.
It was one of the most controversial policies of The Troubles, the three decades of civil unrest between republican Catholic and pro-British Protestant communities.
There were also clashes on Thursday night at an anti-internment bonfire near Belfast city centre, when eight police officers were injured and eight people were arrested.
Last month the city was hit by several nights of rioting, predominately by pro-British loyalist groups.
The 1998 Good Friday agreement, which set up a power-sharing government between republicans and loyalists, largely ended the violence in the British-controlled province although sporadic attacks and bomb threats continue.
