Belfast, Northern Ireland (CNN) -- Fresh pro-British protests erupted Saturday in the Northern Ireland city of Belfast, police said, hours after nine officers were hurt in rioting.
Tensions have been high since city council members there voted a month ago to stop flying the Union flag year-round, restricting it instead to certain days.
Officers came under sustained attack in east Belfast from more than 100 people, some throwing fireworks and bricks, the Police Service of Northern Ireland said Saturday.
The violence continued into the night. Police said they deployed water cannon and fired plastic bullets and that a number of arrests were made.
Authorities were also investigating reports that a number of shots were fired at police lines.
Friday night, more than 30 petrol bombs were thrown at officers during serious disorder in the same part of the city. Officers were also pelted with ball bearings, fireworks and masonry, police said. Eight officers were hurt.
Another officer was hurt Friday in the Newtownabbey area, police said, where bottles and bricks were thrown at police in the course of a protest lasting several hours.
Police have arrested more than 20 people in connection with the disorder.
A lunchtime demonstration outside City Hall appeared to pass without trouble.
Northern Ireland's political leaders have called for an end to the pro-British protests, which were prompted by the vote on the Union flag of the United Kingdom.
The disorder follows a summer of heightened tensions between Northern Ireland's Catholic and Protestant communities. Riots in September left dozens of police officers hurt.
The recent violence follows more than a decade during which Northern Ireland has made steady progress toward lasting peace and stability.