Ten officers have been hurt after masked gangs in east Belfast hurled bricks, bottles and petrol bombs at police in rioting last night.
It was the latest flare-up of violence linked to restrictions on flying the Union flag at City Hall.
A 23-year-old man and a 16-year-old boy have been charged with rioting.
About 100 unionist demonstrators took part in the protest. At one stage, the hooded and masked protesters set a car on fire, blocking access to one road.
At least one officer was briefly taken to hospital.
Protests erupted weeks ago when councillors in Belfast voted to limit the number of days the Union flag could be flown over City Hall to 17 days a year, rather than all year round.
Some of the protests have turned violent.