Violence has erupted for a second straight night in Northern Ireland between Protestants and Catholics.

Police say approximately 500 people were involved in Tuesday fighting that saw both sides throw rocks and gasoline bombs. Police officers in east Belfast struggled to keep the two sides apart.

Riots Monday night left two people shot in the leg and involved members of the Ulster Volunteer Force, a paramilitary Protestant group.

Northern Ireland officials call the two nights of violence the worst outbreak after a long period of quiet.

It is unclear what sparked the attacks which come at the start of the annual sectarian parade season.

Protestants in Northern Ireland want to remain part of the United Kingdom while Catholics want to break free of British rule and join Ireland.

The factions signed the Good Friday Peace Accords in 1998, ending nearly 30 years of often deadly violence. Both sides now govern Northern Ireland as part of a power-sharing agreement.

A report Tuesday by the Historical Enquiries Team, which investigates incidents during the decades of fighting, says the Provisional Irish Republican Army lined up and executed 10 Protestant workers in 1976.

The Provisional IRA denied responsibility for the massacre.