The incident took place on Monday evening while the woman, 20, was going in an auto-rickshaw on Ghodbunder Road in Kapurbawdi area, when the four accused, who were onboard a towing van, began molesting her, Thane Police PRO Sukhada Narkar said.
The van was on duty of the traffic police for towing vehicles that flouted traffic rules, while a traffic constable was also present inside the vehicle.
The four men passed lewd comments and also made objectionable gestures at her, the officer said.
When the woman and the driver of the auto-rickshaw objected to it, the accused hit the rickshaw with the thick stick and also assaulted the victim and the driver.
The woman then raised an alarm and was helped by passers, who intervened and who the accused to the Kapurbawdi police.
The victim in her complaint alleged that the traffic constable did take any action to stop the harassment and remained a moot spectator.
The accused were identified as Rakesh Mohan (30), who was the driver, Shamshad Khan (21), Durvesh Jadhav (23) and Ramesh Patel (30) and were apprehended in the wee hours.
A case has been lodged under IPC sections 354 (outraging modesty of a woman), 324 (voluntarily causing hurt by dangerous weapons), 509 (insult the modesty of a woman), 504 (intent to provoke breach of the peace), 34 (common intention), against the four persons.