The incident took place in Mandi House in central Delhi at around 11 pm yesterday and the victim is a 16-year-old girl.
"The girl had boarded cluster orange bus. When it reached near Mandi House around 11 pm, she was molested by the conductor of another bus who was travelling in the same vehicle," a senior police official said.
He said the girl had left the house angry with her family.
"The girl left her house at around 9:30 pm. The bus reached Mandi House at around 11 pm. A separate case of kidnapping is found registered about the same girl at her home police station. She left the house due to unhappiness with her family," the official said.
The bus was plying between Khayala to Lajpat Nagar.
Ranjit Singh, who was off the duty, was arrested by Constables who were on patrolling near Mandi House.
A case under sections 354 (assault or criminal force to woman with intent to outrage her modesty) and 509 (word, gesture or act intended to insult the modesty of a woman) of Indian Penal Code has been registered against him.