The political group linked to the Real IRA has accused police of harassing its members.
Stephen Moore, a member of the 32-County Sovereignty Movement, said he was approached by two men he believed to be special branch officers at Belfast International Airport while returning from a family wedding in England.
Mr Moore, from Belfast's Falls Road, said they produced photographs of him at the wedding, and asked if he would "set up for assassination" a national executive member from Belfast, Karl Reilly.
"They continued to say that a top Provisional informer had provided them with information that my life was under threat and that if I set up Karl Reilly, this threat would be lifted, " he said.
Francis Mackey, chairman of the national executive, said was a "very serious development" at a press conference yesterday.
"We demand an end to this intimidation and harassment of innocent people and demand the end of this colonial police force," he said.
But Belfast Ulster Unionist councillor Jim Rodgers, chairmn of the city's district policing partnership said the claims were "not surprising".
He said: "We need specialist units and undercover personnel including Special Branch, although they must at all times act within the law."
A police spokeswoman said any allegations of police misconduct should be referred to the ombudsman. Mr Mackey claimed political developments vindicated his party's anti-agreement stance.
Asked about bomb attacks by the Real and Continuity IRA this week, he said: "Republicans will keep focused on what it was that was wrong in the first place that led to conflict – Britain's illegal claim to sovereignty here.
"We in the 32-County Sovereignty Movement have given a peaceful alternative to all of this by going to the UN, lodging a challenge to the British government under international law."
