Dr Sen, an office bearer of the People's Union for Civil Liberties has been fighting to defend the rights of the tribals in the state.
He was held under the Chhattisgarh Special Public Security Act with the government accusing him of helping Naxals in the state.
He has been a critic of the Salwa Judum - the government-sponsored movement to counter the Naxalites and supporters say this is what he's being targeted for and also for indulging in unlawful activities.
There are allegations that Dr Sen's activities are connected to those of Narayan Sanyal, a member of CPI-Maoist, a banned outfit.
Police suspect Sanyal's hand in the storming of a jail in Bihar in which Maoists freed 400 inmates.