Loyalists and nationalists have clashed in east Belfast after a union flag protest in the city centre on Saturday.
It happened as loyalists returned from a demonstration at city hall, against the council’s decision to limit the number of days the union flag is flown.
Rocks and fireworks were thrown at Albertbridge Road, near the nationalist Short Strand. Police used water cannon and fired at least one plastic bullet.
Four officers were hurt in Castlereagh Street. Two were taken to hospital.
The trouble began about 14:30 GMT on Saturday.
Riot police intervened to separate the crowds at the Short Strand interface but the trouble is continuing.
A BBC reporter in the area said officers have moved the loyalist crowd back from the interface, but police are “coming under sustained attack with bricks, bottles and fireworks”.
Earlier, nearly 1,000 people took part in a flag protest at Belfast City Hall to protest.
Loyalist street demonstrations have taken 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, as opposed to 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 in Northern Ireland.
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.
Fireworks, petrol bombs and rocks were used to attack police in Newtownabbey and a bus was set alight near the Rathcoole estate.
A crowd of more than 100 loyalists threw missiles at police in Carrickfergus.
Two arrests were made and of the four police officers who were injured, one required hospital treatment.
Dozens of police officers have been injured in 40 days of protests and more than 100 people have been arrested.
Friday’s protests – most of which passed without violence – took place in Counties Antrim, Armagh, Down, and Londonderry.
However, in County Antrim, police used water cannon during trouble on the Shore Road in Whiteabbey and also in Newtownabbey, where a PSNI Land Rover with a CCTV camera on the roof was set alight with a petrol bomb.
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.
The protesters jeered at the elderly man and refused to let him through.