Thirty two police officers were injured and 11 people arrested during Orange Day parades in Northern Ireland on Friday night and Saturday.
Trouble between the Protestants and Catholics still sometimes flares since a peace deal was signed in 1998, which largely ended three decades of unrest. Much of Belfast remains divided along religious and nationalist lines.
Petrol bombs, bricks, bottles and fireworks were thrown at police, who responded with plastic bullets and water cannon.
A total 32 police officers were injured. Another 400 reinforcements are being brought in from Britain.
Tens of thousands of Orange Order marchers, wearing orange sashes and waving British flags, paraded at more than a dozen venues across Northern Ireland on Friday to mark the 1690 victory at the Battle of the Boyne by Prince William of Orange, a Protestant, over King James of England, a Catholic.
"I think that this morning some of the leadership of the Orange Order need to reflect upon whether they provided the responsible leadership asked for by myself and the party leaders," said Chief Constable Matt Baggott.
Calm returned on Saturday after the Orange Order said it was suspending protest action over a ruling that nmarchers could not use a stretch of road that divides the two communities.