The accused, a 24-year-old carpenter Abhinav Dube, was drunk at the time of the arrest last night, police said.
The incident took place when the constable was on her way to Sarojini Nagar to report for duty. She had boarded a DTC bus around 8 PM from West Delhi's Nangloi area.
Later, Dube also boarded the bus and allegedly started molesting the woman constable.
"Despite several warnings and being told that she was a police official, the accused did not stop misbehaving with her. He even dared her to take action against her," said police, adding that no fellow passenger intervened to stop her ordeal.
She then pulled Dube out of the bus and called a PCR van. An FIR has been registered against the accused and further investigation is going on in the case, police said.
The constable's husband is also employed with Delhi Police.