An official said that Sopan Nivrutti Ugale, a resident of suburban Goregaon, was arrested yesterday by Crime Branch's Unit -12.
The incident occurred when the victim and her family members were travelling by bus from Shirdi to Mumbai on the night of June 13, he said.
The official said that the 6-year-old girl was sexually assaulted by the accused who had boarded the bus from Sangamner in Ahmednagar district.
The accused assaulted the girl while her family members were asleep, the official added.
A team of the Crime Branch's Unit 12 started investigations and managed to zero in on the accused with the help of private bus operators, the official informed.
Ugale was arrested within 12 hours of the incident and charged under relevant sections of the IPC and the Protection of Children from Sexual Offences (POCSO) Act.
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