Indian police on Monday were interrogating a school bus cleaner after his arrest on suspicion of raping a four-year-old in the vehicle on the outskirts of Mumbai earlier this month.

The 26-year-old was arrested at the weekend after the girl told her parents of the incident more than a week after it occurred on September 6, police said.

News of the assault comes days after four men were sentenced to death last week over the fatal gang-rape of a student in the capital New Delhi in December, a brutal attack that led to weeks of protests and outrage over the treatment of women and girls in India.

Financial hub Mumbai, long considered safer for women than New Delhi, has also been recently rocked by the gang-rape of a photographer in August while she was out on assignment in the city with a male colleague.

According to the police complaint filed by the four-year-old's parents, she and her friend were the first to climb on to the bus after classes in the Thane district neighbouring Mumbai.

"The cleaner called her to the back seat on the pretext of showing her some tricks. He then caught the girl and raped her," said inspector S.K. Mohite at the local Badlapur police station.

The girl screamed and her friend went to help her, but the man threatened to kill them if they reported the matter to the school, the inspector said.

"There was no woman attendant on the bus at that time," he added.

Mohite said the girl has undergone medical examination and doctors confirmed she was sexually assaulted.

"Medical details and investigations into the cleaner's past are being gathered," he added.

The outcry over the Delhi gang-rape led to a tougher law in India against rapists, but cases of sexual violence have continued throughout the country.

Five suspects have been arrested over the Mumbai photographer's gang-rape and are expected to be formally charged later this week.