BELFAST—Forty-seven police were injured while trying to separate Catholic and Protestant rioters in Belfast on Sunday after violence broke out during a march by a Catholic band, police in Northern Ireland said Monday.

Police fired water cannon to defend themselves from gasoline bombs, fireworks, stones and bottles thrown at them by both sides in running battles lasting nearly 12 hours, police said.

The initial violence was caused by pro-British Protestant groups demonstrating against the Republican band parade.

The area had been tense for a week, since violence erupted — and seven police officers were hurt — after a Protestant band marched past a Catholic Church, playing music in defiance of a ban by the parades commission, which regulates marches in the British province.

Hundreds of large fireworks and 34 gasoline bombs were thrown at the police as they tried to keep the rival factions apart, Chief Supt. George Clark said.

Clark said he had been angered and saddened by Sunday’s violence by loyalist and republican rioters, which he described as “savage, appalling and reprehensible.”

The police had warned rioters at one point that they might use plastic bullets against them.

Sinn Fein politician Gerry Kelly said the violence on the Protestant side had been orchestrated by the Ulster Volunteer Force and the Ulster Defence Association, two loyalist groups that are supposed to be observing a ceasefire.

Police have urged community leaders to hold talks before more marches, part of the traditional parade season, take place in Belfast later this month.

Paramilitary violence between the province’s mainly Catholic republicans and pro-British Protestants, which had raged on and off for three decades, has largely ended since a peace agreement was signed in 1998. But much of Belfast remains divided along sectarian lines.

Riots often erupt during the summer months when Protestant groups hold traditional parades that are seen as provocative by the nationalists, many of whom want to be part of a united Ireland.