Ms Patkar is the public face of the Narmada Bachao Andolan (Committee to Save the Narmada), that has fought a 20-year struggle against 4 state govts to stop construction of dams along the Narmada river.
Ms Patkar began a fast on 29 March to try to stop the height of the largest dam being raised by 10 metres - a move that would displace 25,000 families in the river valley. In the past her protests have forced the World Bank to cancel funding for the Narmada dam, partly because of the human cost of displacement.
Environmentalists, human rights groups and political parties have condemned her arrest, accusing the government of "stifling legitimate protest". One of her most celebrated supporters, the novelist Arundhati Roy, said that India's govt had put on a "horror show" in arresting Ms Patkar.
Sonia Gandhi, the ruling Congress party president, expressed concern over Ms Patkar's plight.
"The government cannot afford to let anything happen to Medha," said Tarun Tejpal, the editor-in-chief of Tehelka magazine. "This administration came to power because it was supposed to be concerned with the ordinary man, not booming stockmarkets and big dams. She is a symbol standing up to their hypocrisy."