The leader of ITAK's youth wing, Subramaniyam Sivakaran was arrested on Wednesday morning by the Terrorism Investigation Department (TID).
Mr Sivakaran, who is also the editor of a Tamil newspaper called Puthiyavan, was detained at the paper's offices in Mannar and taken to Vavuniya.
The activist, in a recent interview to TamilNet, had condemned the recent arrests and abductions by the TID and compared the Sirisena-regime to that of Mahinda Rajapaksa for its deployment of the Prevention of Terrorism Act.
Mr Sivakaran organised last year's Mullivaikkal commemoration event in Mannar on May 18, and attended the Maaveerar Naal remembrance day on November 27 in the district.
His arrests comes after several Tamils, including former members of the LTTE were detained by the TID over the past few weeks.
May 18 remembrance in Mannar blocked by police (18 May 2015)