BELFAST, Northern Ireland — Northern Ireland police used water cannons to fend off brick-hurling protesters in Belfast on Saturday as violent demonstrations over flying the British flag stretched into a third straight day.

The Police Service of Northern Ireland said it was investigating reports that shots were fired at police lines. A 38-year-old man has been arrested on suspicion of attempted murder, police said.

More than 1,000 demonstrators marched on Belfast’s city hall Saturday afternoon amid a heavy police presence. While the rally passed largely without incident, police then came under attack from a mob of more than 100 people hurling bricks and fireworks. Two men were arrested, police said.

Protesters have been out in force — with sometimesviolent results — since a Dec. 3 decision by Belfast City Council to stop flying the British flag year-round.

Such issues of symbolism frequently inflame sectarian passions in Northern Ireland, where Protestants mainly want to stay in the United Kingdom and Roman Catholics want to unite with the Republic of Ireland.

Many Protestants want the council to reverse its decision about the flag, and dozens of police have been injured in ensuing demonstrations.

Saturday’s flare-up followed a tense Friday night in Belfast when nine police officers were injured and 18 rioters arrested. Police said more than 30 gasoline bombs were thrown at officers, along with ball bearings, fireworks and bricks as they responded to clashes in Protestant sections of the city.