The coordinator of the 'Founding Parliament' initiative, Aram Hakobyan, was detained on Apr. 8 in the Armenian city of Gyumri, News.am reported.
The activist is suspected of illegal acquisition and possession of weapons.
An investigation is underway.
Previously, Armenia's Investigative Committee announced the arrest of four people as part of an investigation of a criminal case, launched on the preparation of mass unrests in places of public events for April 24.
Varuzhan Avetisyan, Pavel Manukyan, Gevorg Safaryan, and Zhirayr Sefilyan were detained on suspicion of committing the crime.
All of them are members of the opposition movement, known as the 'Founding Parliament'.