Twenty-nine police officers have been injured in rioting in Belfast following a loyalist protest over the union flag.
Officers fired six baton rounds and used water cannon during the 40th day of street protests.
The latest unrest began around 14:30 GMT, when loyalists and nationalists clashed at a sectarian interface at Short Strand, east of the city.
PSNI Chief Constable Matt Baggott said officers had used "exceptional courage" to bring the situation under control.
Demonstrators were passing the nationalist Short Strand area - after a 1,000-strong protest outside Belfast City Hall against the council's decision to restrict the flying of the Union flag - when the violence broke out.
Bricks, stones and missiles were thrown at police and four officers were taken to hospital with injuries - two were later discharged.
A car was later set on fire at the junction of Castlereagh Street and Templemore Avenue.
The violence subsided in the early evening but after 21:00 GMT it erupted as loyalists attacked police again at Castlereagh Street. Police used water cannon on rioters to push them away from the interface with the Short Strand.
Chief Constable Matt Baggott said: "This was a difficult operation dealing with a large number of people determined to cause disorder and violence.
"My colleagues brought the situation under control with exceptional courage and professionalism. I know the vast majority of people will be grateful for their efforts.
"Police will continue to engage with all those committed to finding a solution to these issues."
Loyalist street demonstrations have been taking place for almost six weeks, since Belfast City Council voted to change its longstanding union flag policy on 3 December.
The council, which now has a nationalist majority, voted to fly the flag at Belfast City Hall on a number of designated days, rather than every day of the year.
The majority of the street demonstrations have passed without incident, but some have resulted in serious rioting.
On Friday night, four police officers were injured during union flag protests, one was taken to hospital.
The most serious violence took place outside Belfast - in Newtownabbey and Carrickfergus in County Antrim. Officers fired five plastic bullets as rioters threw more than 30 petrol bombs.
In Rathcoole in Newtownabbey, a distressed pensioner pleaded with protesters to let him pass through a road block so he could make his way to visit his seriously-ill wife in hospital.