JOIN USIt is also "likely to enflame tensions in the conflict-affected area" of Chhattisgarh, Amnesty said in a statement.
"Life in prison is an unusually harsh sentence for anyone, much less for an internationally recognized human rights defender who has never been charged with any act of violence," said Sam Zarifi, Amnesty's Asia-Pacific director.
"State and federal authorities in India should immediately drop these politically motivated charges against Sen and release him," it said.
Sen, who is also a medical doctor, was convicted Friday of sedition and conspiracy under the Chhattisgarh Special Public Safety Act, 2005, and the Unlawful Activities Prevention Act, 2004.
He was immediately taken into custody, having been out on bail since May 2009.
"Sen, considered a prisoner of conscience by Amnesty International, was convicted under laws that are impermissibly vague and fall well short of international standards for criminal prosecution," Zarifi said.
"Instead of persecuting Sen, authorities in Chhattisgarh should be acting to protect the people of the region from the abuses committed by the Maoists as well as state security forces and militias.
"This sentence will seriously intimidate other human rights defenders who would provide a peaceful outlet for the people's grievances, especially for the indigenous Adivasi population," Zarifi said.
Sen is a pioneer of health care to marginalized and indigenous communities in Chhattisgarh, where the police and armed Maoists have been engaged in clashes over the last seven years.
Sen was detained without proper charges for seven months, denied bail and kept in solitary confinement for three weeks.
Amnesty said that many of the charges against him stem from laws that contravene international standards. "Repeated delays in the conduct of his trial have cast doubts about its fairness."
Amnesty has repeatedly called on Indian authorities to immediately drop all the charges against Sen.