BELFAST, Northern Ireland -- Authorities in Northern Ireland sought to restore calm Thursday after Protestant and Catholic young people in Belfast hurled bricks, fireworks and gasoline bombs at police and one another. It was the worst mayhem in a week of street violence in the region, where Britain's exit from the European Union has unsettled an uneasy political balance.
Crowds including children as young as 12 or 13 clashed across a concrete "peace wall" in west Belfast that separates a British loyalist Protestant neighborhood from an Irish nationalist Catholic area. Police fired rubber bullets at the crowd, and a city bus was hijacked and set on fire.
Northern Ireland has seen sporadic outbreaks of street violence since the 1998 Good Friday peace accord ended "the Troubles" -- decades of Catholic-Protestant bloodshed over the status of the region in which more than 3,000 people died.
But Jonathan Roberts, police service assistant chief constable, said Wednesday's mayhem "was at a scale we have not seen in recent years." He said 55 police officers had been injured over several nights of disorder and it was lucky no one had been seriously hurt or killed.
Britain's split from the EU has highlighted the contested status of Northern Ireland, where some people identify as British and want to stay part of the U.K., while others see themselves as Irish and seek unity with the neighboring Republic of Ireland, an EU member.
Unrest has flared over the past week -- largely in loyalist, Protestant areas -- as tensions rise over post-Brexit trade rules and relations worsen between the parties in the Protestant-Catholic power-sharing Belfast government.
U.K. Prime Minister Boris Johnson condemned the unrest, saying "the way to resolve differences is through dialogue, not violence or criminality." He sent Northern Ireland Secretary Brandon Lewis to Belfast for talks with the region's political leaders.
Meanwhile, Northern Ireland's Belfast-based assembly and government held emergency meetings Thursday and called for an end to the violence.
First Minister Arlene Foster of the pro-British Democratic Unionist Party warned that "Northern Ireland faces deep political challenges ahead."
"We should all know that when politics are perceived to fail, those who fill the vacuum cause despair," said Foster, who heads the Northern Ireland government.
Deputy First Minister Michelle O'Neill of Irish nationalist party Sinn Fein called the violence "utterly deplorable."
Despite the united message, Northern Ireland's politicians are deeply divided, and events on the street are in many cases beyond their control.
As many predicted, the situation has been destabilized by Britain's departure from the EU -- after almost 50 years of membership -- that became final Dec. 31.Gallery: Northern Ireland unrest
A post-Brexit U.K.-EU trade deal has imposed customs and border checks on some goods moving between Northern Ireland and the rest of the U.K. The arrangement was designed to avoid checks between Northern Ireland and Ireland because an open Irish border has helped underpin the peace.
But unionists says the new checks amount to the creation of a border between Northern Ireland and the rest of the U.K. -- something they fear undermines the region's place in the United Kingdom.
The latest disturbances followed unrest over the Easter weekend in pro-British unionist areas in and around Belfast and Londonderry, also known as Derry, in which cars were set on fire and projectiles and gasoline bombs hurled at police officers.
Some politicians and police have accused outlawed paramilitary groups of inciting young people to cause mayhem.