Sinn Fein president Gerry Adams missed a St Patrick's Day event in the US after being delayed at a Washington airport for "secondary screening".
Gerry Adams had been attending meetings in Washington
He had been invited by Congressman Brian Higgins to speak in Buffalo.
Homeland Security officials said he had a routine check and "secondary screening" but was not arrested.
Mr Adams said while he understood "the need for vigilance, Sinn Fein members are the victims of an unacceptable and unfair administrative practice".
He said that he had protested about the delay at Dulles Airport to the US administration.
Mr Higgins told the audience Mr Adams was absent because his name was on a "terror watch list", but Homeland Security officials would not comment on this.
Mr Adams, who had earlier attended a St Patrick's Day event at the White House, was due to speak at the Buffalo Irish Center in New York state.
AP news agency said Congressman Higgins told the crowd awaiting Mr Adams' speech: "When I spoke with his assistant a little while ago, their luggage was still being, let's just say, inspected.
"Gerry Adams should not have been on a terror watch list," he said.
"That his name would appear in this untoward manner and that appearance ruined his ability to celebrate St Patrick's Day in Buffalo is sad and unfortunate when one considers Gerry Adams' leadership in the decommissioning of the IRA's weapons and his commitment to the Good Friday agreements," Reuters quoted him as saying.
A Homeland Security official refused to comment on whether Mr Adams' name had appeared on a terror watch list, saying privacy restrictions precluded them from discussing individuals on no-fly lists, said AP.
The official also declined to explain why Mr Adams underwent the secondary screening, which was described as "thorough".