LONDON » Six months after the British government handed control of the police in Northern Ireland to local officials, the worst rioting in years has left 82 police officers injured in Belfast, the provincial capital, including a female officer who was hospitalized in stable condition after a concrete slab was dropped on her head from a rooftop.

The rioting reached a peak Monday in the northern Belfast district of Ardoyne, set off by one of the most emotive occasions in the calendar, the annual July 12 marches by the Orange Order. The order, a Protestant fraternal organization, has been a bulwark of Protestant—and British—supremacy in the six northern counties of Ireland. It stages the marches on the anniversary of the Battle of the Boyne in 1690, when the victory by the Protestant English king, William of Orange, secured British dominion in Ireland for more than 200 years.

Hundreds of rioters in Ardoyne battled police with gasoline bombs, bricks, metal bars and planks on Monday after police officers in riot gear moved in to remove a group of about 100 protesters who had staged a sit-in to prevent the Orange Order marchers from making their return passage through a predominantly Roman Catholic neighborhood.

The confrontation was a throwback to the violence that erupted regularly during the Orange Day parades in the years before the 1998 Good Friday agreement, which set a blueprint for peaceful settlement of the enmities between the mainly Protestant unionists who seek to keep the province a permanent part of Britain, and the mainly Catholic republicans, who want a united Ireland. It led eventually to the establishment of the power-sharing government that has ruled in Belfast since 2007.

The parade issue lingered, though, and erupted anew last week when the Orange Order rejected a new system for mediating the routes and timing, raising fresh anger, particularly among republicans.

That was followed by the rioting over the past 48 hours in Belfast, along with lesser disturbances during Orange marches in the cities of Londonderry and Lurgan, that police officials said was provoked by so-called "dissident" republicans opposed to power-sharing. Reporters at the scene confirmed having seen members of breakaway factions of the Irish Republican Army moving among the protesters, many of whom were youths from Catholic districts in Belfast, where there are crushing rates of unemployment.

The assistant chief constable Alistair Finlay, charged with oversight of the police units assigned to contain the rioting, spoke out against what he described as the failure of the unionist and republican leaders of the power-sharing government, Peter Robinson and Martin McGuinness, to intervene to stop the Ardoyne confrontation, a failure he said had left the police to "form a human barrier attempting to keep the peace."

Later on Tuesday, the two men, respectively first minister and deputy first minister in the government, issued a statement condemning what they called "outright thuggery and vandalism" by the rioters. "This will require the community to stand united against all those forces seeking to bring conflict back to our streets," McGuinness said.