Jakarta police spokesman Argo Yuwono said Muhammad Al Khathath, leader of the Muslim Peoples Forum umbrella group, was suspected of involvement in a conspiracy to commit treason and was arrested at a hotel today.
The forum, which includes the Islamic Defenders Front vigilante group, has been behind a series of huge protests against the minority Christian governor of Jakarta who is on trial for allegedly blaspheming the Quran.
The protests have shaken mainstream Muslim groups and the secular government of Muslim-majority Indonesia.