Environmental activist T Mugilan, who went missing five months ago, was detained by the state police at Tirupati railway station on Saturday.
In a press conference in February, the activist had alleged involvement of top police officials in the violence that occurred in the anti-Sterlite protest in Thoothukudi. He went missing the same day.
In May last year, 14 people were killed and more than 60 were injured in the alleged police firing during the violent protest against Sterlite plant in Thoothukudi.
The people had protested against the construction of the Sterlite copper plant, claiming that it was polluting groundwater, and was hazardous for the environment.
Taking note of the people's demand, the Tamil Nadu government had on May 28 ordered the closure of the plant.
In August, the Madras High Court ordered a Central Bureau of Investigation (CBI) probe into the police firing.