Omagh:Independent republican candidate Gerry McGeough was arrested outside the count centre in Omagh yesterday.
Mr McGeough, a former member of Sinn Féin and a candidate in the Fermanagh-South Tyrone constituency, was taken away by the PSNI after being arrested in the car park of Omagh Leisure Centre.
Mr McGeough was convicted of gun running on the German-Netherlands border in 1988 and was imprisoned in Germany and later in the US.
He left Sinn Féin because he said it had become too liberal on social issues.
A statement from the PSNI said: "A 46-year-old man has been arrested by police in Aughnacloy. He was arrested as part of an investigation into serious terrorist crimes. He has been taken for questioning to the serious crime suite at Antrim Police station".
The DUP's Arlene Foster - who topped the poll in the constituency - seized on the incident to demand Sinn Féin's response. "What have they got to say about that? What's their attitude to that?"
This was a simple matter of law and order, she said.
In Belfast later, Sinn Féin leader Gerry Adams said Mr McGeough should be set free.
"He was campaigning openly and attended one of our meetings. He should be released," he added.
Sinn Féin MP Michelle Gildernew also called for Mr McGeough's release, describing her republican rival's arrest as an example of "political policing". The Fermanagh-South Tyrone MP said: "Gerry lives openly in the area, so there can only be one interpretation put on the decision to arrest him in this way.
"This is one of the worst examples of political policing and one of the reasons we have to get rid of the people that are involved in creating this kind of mayhem on the day of an election," she added.