The union secretary of Amma Makkal Munnetra Kazhagam, Saravanan, was hacked to death by unidentified persons in Tamil Nadu's Sivaganga Sunday, police said.
Four special teams have been constituted to arrest the culprits, they said.
Saravanan was killed when he was taking his morning walk, police said, adding the motive behind the killing was being investigated.
Police said AMMK leaders and activists, including district secretary Umadevan, refused to accept Saravanan's body in protest. They demanded the culprits be arrested.
Superintendent of Police Jayachandran assured the AMMK men swift would be taken action in the matter.
Police personnel were deployed at the government hospital where the AMMK leader's body was kept.
The police official said 20 people have been detained in connection with the killing.