"We rescued the woman and admitted her to a local hospital. She has received head injuries. She is critical," Additional District Superintendent of Police G.C. Mallick told Thomson Reuters Foundation.
"I have never witnessed such an inhuman incident during my three decades of service."
Police said they were investigating Sunday's attack, during which the woman was tied up for around 12 hours, but no arrests had been made.
The practice of branding women as witches and assaulting or killing them is still common is some parts of India, particularly among tribal communities, despite there being a law against it.
According to government statistics, there were 160 cases of murder linked to witch hunts in 2013, and 119 the previous year.
Earlier this month, a woman in Bihar was beaten to death after villagers accused her of practising witchcraft.