The incident took place in Tura, the district headquarters of West Garo Hills.
The 15-year-old girl was allegedly molested by five labourers on Friday evening. Three were apprehended by the mob, while two managed to flee.
"Curfew has been imposed in West Garo Hills district from 10pm after trouble erupted after a mentally challenged minor girl was allegedly molested by five labourers," principal secretary in charge home, KS Kropha, said.
He said curfew was imposed to prevent the situation from escalating.
Police said they arrested Md Raju Allam, Nehar Ali and Babul Sheik, who had allegedly molested the minor girl. All three have been admitted in Tura civil hospital amid tight security.
Police also opened blank fire to disperse mobs attempting to storm the hospital to lay their hands on the three accused, the official said.