The instructor used to take karate training classes at his residence in Kalyan township of the district, the official at the Thane district rural police control room said.
On December 24 last year, the teacher asked the 11-year-old girl to lie down on a mat on the pretext of imparting training to her. He then allegedly touched her inappropriately and sexually assaulted her, he said.
Initially, the girl did not reveal about the incident to anyone out of fear but later informed her mother, who lodged a complaint with the police late last night, the official said.
The Kalyan taluka police arrested the accused in the early hours today and booked him under IPC section 376 (rape) and relevant provisions of the Protection of Children from Sexual Offences (POCSO) Act, he added.