In a statement the party said that Christy O’Kane, who sits on Saoradh’s national executive, was held by members of the France’s General Directorate for Internal Security (GDIS) in an airport during a trip to Paris last week.
He claims he was brought to meet a man and a woman who introduced themselves as MI5 officers.
Saoradh, the political wing of the New IRA said Mr O’Kane, who was jailed in 2015 for Provisional IRA terrorist offences, was eventually released after “a loud verbal altercation”.
The Derry man’s detention came after members of Saoradh held a protest at MI5’s headquarters in Holywood, Co Down earlier this month.
It is claimed that Mr O’Kane’s partner later received a text message, believed to be from MI5, when the pair returned home.
In the message the author wrote “just so there is no confusion, what we are asking him to do is use his historical knowledge to stop the youngsters in Derry throwing away their lives”.
The MI5 official added that the agency is “not looking for him to work with us, our message was, ‘don’t get drawn into violence again’.
“We hope you can influence him more than we might be able to.”
Saoradh spokesman Paddy Gallagher said: “For the past number of years we have seen an increase in this type of activity particularly contact being made to family and friends of republican activists.”