BELFAST — A week of rioting reached new heights of intense violence in Northern Ireland with clashes overnight Wednesday that saw demonstrators hijack and torch a double-decker bus and battle police, who responded with plastic bullets.

Police and politicians called it the worst violence in Northern Ireland in more than a decade, triggered in part by pro-British Protestant unionists who fear that their home is drifting away from Britain in the new realities of a post-Brexit world.

When the nightly riots began across Northern Ireland a week ago, it was mostly young men and teenagers from unionist neighborhoods skirmishing with police. Some of those arrested were 13 and 14 years old.

But on Wednesday, hundreds of Irish nationalist and unionist demonstrators began to face off, a worrying escalation that stirred old memories of the 30 years of sectarian violence known as the Troubles, which led to the deaths of 3,500 civilians, British security personnel and paramilitary members.

On Thursday night, images on Twitter showed demonstrators gathering again in Belfast — some setting trash cans on fire and blocking streets — and police deploying water cannons.

Saturday will mark the 23rd anniversary of the Good Friday Agreement, brokered by the United States, which stopped decades of civil strife between Catholics and Protestants, and brought an end to the militarization of the borderlands between Northern Ireland and the Republic of Ireland to the south.

At one of the “peace walls” separating traditionally Catholic and Protestant neighborhoods in Belfast on Wednesday night, the two sides hurled bricks and shot fireworks at each other across the barrier.

One poignant image showed a masked young man throwing a firebomb across a locked gate that was covered with an old mural that read: “There was never a good war or a bad peace.”

Ireland’s foreign minister, Simon Coveney, appealed for calm and warned, “This needs to stop before somebody is killed or seriously injured.”

“These are scenes we haven’t seen in Northern Ireland for a very long time. They are scenes that many people thought were consigned to history, and I think there needs to be a collective effort to try to defuse tension,” he told the Irish national broadcaster RTE.

Dozens of police officers have been injured. Politicians representing unionist and nationalist sides have condemned the violence, even as they traded blame for the rioting.

On Thursday, the leaders from both sides in the Northern Ireland Assembly issued a joint statement, expressing support for law and order and the police as well as grave concerns about “the scenes we have all witnessed on our streets.”

Heightening the anxiety, it appears that paramilitary groups are now directing some of the violence, said Jonathan Roberts, assistant chief constable for the Police Service of Northern Ireland.

“Last night was at a scale we haven’t seen in Belfast or further afield in Northern Ireland for a number of years,” Roberts told reporters. “We are very, very lucky no one was seriously injured or killed last night, given in particular the large number of petrol bombs thrown.”

British Prime Minister Boris Johnson said he was “deeply concerned” by the clashes, especially attacks on police, a bus driver and a photojournalist. “The way to resolve differences is through dialogue, not ­violence or criminality,” he said Wednesday night.

Members of the U.S. Congress and President Biden have warned Johnson that Brexit must not undermine the peace in Northern Ireland. On Thursday, White House Press Secretary Jen Psaki called for calm. “We remain steadfast supporters of a secure and prosperous Northern Ireland in which all communities have a voice and enjoy the gains of the hard-won peace,” she said, adding: “We welcome the provisions in both the E.U.-U.K. Trade and Cooperation Agreement and the Northern Ireland Protocol, which help protect the gains of the Belfast Good Friday Agreement.”

The sources of the week’s violence are varied, stirred by grievances old and new.

Unionists and their politicians, who feel themselves a part of Britain, dedicated to queen and country, are upset with the Northern Ireland Protocol agreed upon by Johnson and his counterparts in the European Union to seal their Brexit deal.

The emerging realities of trade after Brexit have given rise to complaints that Northern Ireland is being treated differently than Britain, with the movement of goods effectively controlled by new customs and controls across the Irish Sea.

Unionists were also upset that up to 2,000 people came out to watch the funeral cortege in June for Bobby Storey, a former intelligence chief for the Irish Republican Army and a top figure in the nationalist Sinn Fein party.

Arlene Foster, leader of the Democratic Unionist Party, demanded the resignation of the chief of police for failing to stop the gathering, which broke covid-19 lockdown restrictions.

“Reasons for current anger are very complex,” said Justice Minister Naomi Long, leader of the Alliance Party, which occupies the middle ground in Northern Ireland. “They are related to frustrations and a sense of betrayal around Brexit, around anger concerning the handling of republican funerals, and there is anger directed at police undermining loyalist paramilitaries.”

She said, “We have reached a point where all those things have come together in a toxic mix.”

Journalists on the ground in Belfast also say that many of the young people are bored, angry and deprived, and come out for the excitement and violence of a street skirmish, a phenomenon they call “recreational rioting.”

Eileen Weir, who works on cross-community relations and is coordinator at the Women’s Center in Shankill, in the heart of loyalist Belfast, said: “If you feel identity is being taken away from you, how many thorns can you take? But going to the streets isn’t the answer. We saw the light 23 years ago with the Good Friday Agreement.”

Weir said, “Our youth are being dragged into this and labeled for the future over the actions of handfuls of youth, many not from the areas where violence is taking place.” She said, “We need politicians to come out with solutions, not more problems.”

Clare Bailey, leader of the Green Party in Northern Ireland, said, “We have arrived at this point because we have invested in a political process over a peace process.”

The Good Friday Agreement requires shared power in the Northern Ireland Assembly and government, an uneasy truce that has often led to paralysis.

“We have embedded sectarian division into the framework of our institutions,” Bailey said. “It is time for all actors to recommit to the Belfast Good Friday Agreement in all its parts and deliver it with people and community at the heart.”

Booth reported from London.