BELFAST, Northern Ireland — They call it “the Betrayal Act.”
As British lawmakers prepared to debate Prime Minister Boris Johnson’s new Brexit deal in Westminster on Monday, hundreds of irate unionists in Northern Ireland poured into the East Belfast Constitutional Social Club to plan how they would resist the agreement, should it eventually become the law of the land.
The assembly of rival factions was the first of its kind in 20 years, unionists say, since the signing of the Good Friday Agreement in 1998.
Protestant unionists, who favor preserving the political union between Northern Ireland and Britain, vehemently reject any Brexit arrangement that separates their territory from the rest of the United Kingdom. Mr. Johnson’s new proposal, which would take Britain out of the European Union but leave Northern Ireland effectively in the bloc’s customs union and single market, does just that, they say, drawing a border down the Irish Sea.
But what may rankle most, in the unionists’ view, is the stab in the back by Mr. Johnson, who once promised — as had his predecessor, Theresa May — that no “British government could or should” sign off on a plan that divided Northern Ireland from the rest of the United Kingdom.
“The community is livid,” said Jamie Bryson, a prominent unionist activist who is challenging the Brexit agreement in court, claiming that it breaches the consent mechanism of the Good Friday Agreement. “We feel betrayed, and the general consensus in the room on Monday was that we are sick of this one-sided peace process and we will not tolerate an economic united Ireland.”
The issue was raised in Parliament on Thursday by Nigel Dodds, the deputy leader of the territory’s Democratic Unionist Party, Mr. Johnson’s putative partner in his minority government. Mr. Dodds warned the government that the Brexit agreement posed a threat to “political stability by what you are doing to the unionist community,” adding: “Please wake up. Don’t plow ahead regardless.”
For 30 years before the Good Friday peace accord, generally Protestant unionist militias fought a dirty guerrilla war with the Irish Republican Army, the militant wing of the Catholic republican movement, which favors unification with the Republic of Ireland. More than 3,600 people died in the struggle. Since 1998, Protestants and Catholics have shared power, in a cold peace that has left Northern Ireland largely a grim and economically deprived enclave.
Throughout the Brexit process, pains were taken to avoid a hard border with physical checks between Northern Ireland, which is part of the United Kingdom, and the south, which is in the European Union, fearing that it would inflame latent tensions and undermine the peace agreement.
And that, the unionists warn, is just what Mr. Johnson’s Brexit withdrawal agreement will do.
“We have been completely shafted, and all our concerns have been ignored,” said Bryan Bailey, 58, who attended Monday’s meeting, which was closed to the news media. “We are being treated like second-class citizens and our economy is being sacrificed for Brexit, while everyone on the mainland will be spared.”
Feelings of anger, betrayal, defiance and disappointment were all palpable at the meeting, Mr. Bailey said. Around 400 people packed into two floors of the social club, while hundreds stood outside and listened through the open windows as community members passed around a microphone for people to voice their concerns.
“Loyalism agreed to the peace process on the basis that the union would be safe, and obviously now there is concern that this is being put in the bin to facilitate the Irish government,” Mr. Bryson said.
While unionist representatives said they were committed to a peaceful solution and would pursue political and legal means to frustrate any new Brexit arrangements, some expressed fears that the deal could result in tumult and a resurgence of violence.
“Nobody wants to see violence, and nobody wants to go back to conflict,” Mr. Bryson said. “But people are being pushed into a corner, and they argue that if republicans threatened violence against a hard land border and they made concessions, then maybe if we threaten with violence they’ll get rid of the Irish Sea border.”
That concern was echoed by the local authorities. “Depending on the outcome of what happens with Brexit in the coming weeks or months, should it impact the union, you can anticipate a lot of emotion in loyalist communities and the potential for civil disorder,” Simon Byrne, the head of Northern Ireland’s police service, said in an interview with the BBC’s “Newsnight” program.
“Our concern is that the loyalist community has at times shown it can mobilize quickly, bring large numbers of people onto the streets and engage in public disorder in support of their cause,” Mr. Byrne said.
Even before the Brexit deal took shape, some noted, the level of violence had been rising steadily, particularly among disaffected young people.
Paramilitary-style punishment shootings and beatings have risen 60 percent over the past four years, according to police figures, as dissident groups have tried to take over policing and exert control over deprived communities.
But the bulk of the violence has broken out in republican areas.
Attacks by republican dissidents targeting the police have increased since January, when the New I.R.A. paramilitary group detonated a bomb outside a courthouse in the border city of Derry, also known as Londonderry.
Since then, the group has launched several mortar attacks against police vehicles and sent explosive packages to locations across Britain, including London City Airport, Waterloo train station in London and the University of Glasgow.
In April, a gunman from the group shot and killed Lyra McKee, a local journalist, during a standoff with the police. Paddy Gallagher, a spokesman for Saoradh, a group considered to be the political wing of the New I.R.A., described Ms. McKee’s death as a “tragic event,” but stopped short of condemning the gunman’s actions.
“We need to keep the fight alive,” said a republican dissident member who witnessed the shooting, speaking on the condition of anonymity out of fear of being prosecuted under terrorism laws. “Whether Brexit gives us a hard border or a soft border, we are still under British occupation, and as long as the police are raiding our houses and patrolling our streets with guns, we will not put down our weapons.”
While republican leaders condemn the violence, they say the dissidents are more effective at addressing crime than are the Police Service of Northern Ireland, who they say take too long to respond and are dismissive of petty crime and antisocial behavior.
Analysts say that one of the biggest issues with Brexit negotiations is the British government’s failure to understand the complexities of identity and sectarianism in Northern Ireland.
“Britain has never really understood Ireland, or the mentality in terms of identity on both sides of the community,” said Michael Doherty, a mediator in Derry, one of Northern Ireland’s most troubled cities.
“The peace agreement in 1998 was never clear as to what was agreed,” he said. “It was all about getting to the end of violence, which has had a varied measure of success. It simply transformed the violence to be less violent, and we are at a stage now where that violence is starting to escalate again.”