Women and children were among the passengers on the ferry, which sank in the fast-flowing Brahmaputra river in Assam state on Monday.
“The fate of around 100 others is not known,” PC Haloi, police chief of Dhubri district, said.
The AFP news agency reported that about 150 people were either rescued or swam to safety after the double-decker ferry capsized, police said.
Police and paramilitary soldiers have been sent to the area to help with rescue efforts.
The ferry was on its way from Dhubri, around 300 kilometers (186 miles) from Assam’s largest city Guwahati, to Fakirganj.
Manmohan Singh, Indian prime minister, called the sinking a “tragedy” and said he had pledged all possible assistance to the state government in the search.
Last October, nearly 80 people died when an overcrowded boat sank in the eastern state of West Bangal.