Anti-methane Project Movement leader Jayaraman, his wife Chitra, and two other activists Prithviraj and Arulnesan were detained by the police at Nannilam, in Tiruvarur district on Monday.
The police held them when they arrived to stage an unannounced agitation against the ONGC operations at Nannilam.
The four were immediately taken to the Nannilam police station for interrogation.