(CNN) -- Northern Ireland's top police officer on Saturday condemned violent clashes that left 56 of his officers injured as "mindless anarchy" -- and warned that many arrests will follow.
The disorder broke out in central Belfast as loyalists held counter-protests Friday evening against a nationalist march.
The Police Service of Northern Ireland said Friday that 26 officers had been injured, with five of them needing hospital treatment. But that tally more than doubled on Saturday.
Seven arrests have been made so far, police said.
Warning of more to come, Chief Constable Matt Baggott said, "Significant custodial sentences will be handed down in the weeks and months that follow -- the prisons will be bulging, sadly."
He said "intense violence" had been directed toward the police, and that they had shown "immense courage" in standing their ground to stop the anarchy spreading.
"I know the majority of the population will stand with me in condemning those who scarred the reputation of our beautiful city last night," he said. "Those people had no intention of peaceful protest. They lack self respect and they lack dignity."
Police were braced for further trouble on Saturday.
More parades are scheduled to take place across the country, organized by the Apprentice Boys, a Protestant group. The main demonstration is in Londonderry.
The disorder broke out in Belfast's main shopping district, as loyalists blocked the intended route of the nationalist march, which was eventually redirected.
Assistant Chief Constable George Hamilton, who led the policing operation, said about 1,200 people had gathered "and it was evident that many of them had violent intent."
His officers were pelted with whatever came to hand, he said, including paving slabs pulled up from the street, bricks and lengths of scaffolding.
"There was nothing lawful about this protest. There was nothing peaceful about it. It was sheer thuggery," he said.
Plans for peaceful protests had been registered with the Northern Ireland Parades Commission, which rules on which marches are allowed to take place and which are banned, in an effort to keep friction to a minimum.
Friday's pro-Irish nationalist march followed heightened tensions during some loyalist marches this summer.
Hundreds of parades take place across Northern Ireland each year, the majority involving the Protestant Orange Order and associated organizations.
Most parades pass off peacefully, but when members of one community march near or through neighborhoods dominated by another, violence sometimes results.