BELFAST, Northern Ireland (CNN) -- Fresh pro-British protests erupted Saturday in the Northern Ireland city of Belfast, police said, hours after nine officers were injured in rioting.

Tensions have been high in Belfast since city council members there voted a month ago to stop flying the Union flag year-round, restricting it instead to certain days.

More than 30 petrol bombs were thrown at officers during serious public disorder in east Belfast on Friday night, the Police Service of Northern Ireland said. They were also pelted with ball bearings, fireworks and masonry, injuring eight officers, police said.

Another officer was hurt in the Newtownabbey area, police said, where bottles and bricks were thrown at police in the course of a protest lasting several hours.

Eighteen arrests were made in connection with the disorder.

Saturday's violence was again focused on east Belfast, police said.

A lunchtime demonstration outside Belfast City Hall, in the city center, appeared to pass off without trouble.

Northern Ireland's political leaders have called for the pro-British protests, which were prompted by the vote on the Union flag, to end.

The disorder follows a summer of heightened tensions between Northern Ireland's Catholic and Protestant communities. Riots in September left dozens of police officers injured.

The recent violence follows more than a decade during which Northern Ireland has made steady progress toward lasting peace and stability.

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