They said the woman, who works with an NGO in RK Puram, approached them with a complaint.
She alleged that while she was travelling on a DTC bus (number 711) from Janakpuri to Satya Niketan yesterday, a man made "vulgar gestures" at her, they added.
The police said they stopped the bus before the man could get down. He was detained and found to be drunk. A medical examination was also conducted on him, they said.
The woman alleged that she was made to wait at the police station for over two hours before her complaint was registered. However, Milind Dumbere, Deputy Commissioner of Police (Southwest), said they have not received a complaint from her about the delay.
Police said that a case has been registered and further investigation is underway.