BELFAST, Northern Ireland (AP) — Young people set a hijacked bus on fire and hurled gasoline bombs at police in Belfast in at least the fourth night of serious violence in a week in Northern Ireland, where Britain's exit from the European Union has unsettled an uneasy political balance.

People also lobbed bricks, fireworks and gasoline bombs Wednesday night in both directions over a concrete “peace wall” that separates Protestant, British loyalist and Catholic, Irish nationalist neighborhoods.

Police Service of Northern Ireland Assistant Chief Constable Jonathan Roberts said several hundred people gathered on both sides of a gate in the wall, where “crowds ... were committing serious criminal offenses, both attacking police and attacking each other.”

He said a total of 55 police officers have been injured over several nights of disorder.

The recent violence, largely in loyalist, Protestant areas, has flared amid rising tensions over post-Brexit trade rules for Northern Ireland and worsening relations between the parties in the Protestant-Catholic power-sharing Belfast government. Britain’s economic split from the EU last year has disturbed the political balance in 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, which is an EU member.

U.K. Prime Minister Boris Johnson condemned the unrest, and Northern Ireland’s Belfast-based government was holding an emergency meeting Thursday on the violence.

Johnson appealed for calm, saying “the way to resolve differences is through dialogue, not violence or criminality.” Northern Ireland First Minister Arlene Foster, of the pro-British Democratic Unionist Party, and Deputy First Minister Michelle O’Neill, of Irish nationalist party Sinn Fein, both condemned the disorder and the attacks on police.

The latest disturbances followed unrest over the Easter long weekend in pro-British unionist areas in and around Belfast and Londonderry, also known as Derry, that saw cars set on fire and projectiles and gasoline bombs hurled at police officers.

Authorities have accused outlawed paramilitary groups of inciting young people to cause mayhem.

"We saw young people participating in serious disorder and committing serious criminal offenses, and they were supported and encouraged, and the actions were orchestrated by adults at certain times,” said Roberts, the senior police officer.

A new 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 process built on the 1998 Good Friday accord.

That accord ended decades of violence involving Irish republicans, British loyalists and U.K. armed forces in which more than 3,000 people died. But unionists say the new checks amount to a new border in the Irish Sea between Northern Ireland and the rest of the U.K. They fear that undermines the region's place in the United Kingdom and could bolster ties with the Irish Republic, strengthening calls for a united Ireland.

Both Britain and the EU have expressed concerns about how the agreement is working, and the Democratic Unionist Party, which heads the Northern Ireland government, has called for it to be scrapped.

Katy Hayward, a professor at Queen's University Belfast and senior fellow of the U.K. in a Changing Europe think tank, said unionists felt that “the union is very much under threat, that Northern Ireland’s place is under threat in the union and they feel betrayed by London.”

Unionists are also angry at a police decision not to prosecute Sinn Fein politicians who attended the funeral of a former Irish Republican Army commander in June. The funeral of Bobby Storey drew a large crowd, despite coronavirus rules barring mass gatherings.

The main unionist parties have demanded the resignation of Northern Ireland’s police chief over the controversy, claiming he has lost the confidence of their community.

“You have a very fizzy political atmosphere in which those who are trying to urge for calm and restraint are sort of undermined,” Hayward said.

___

Lawless reported from London.