Dozens of activists including men wearing bras staged a protest in Hong Kong on Sunday after a woman was sentenced to three-and-a-half months' jail for assaulting a police officer with her breast.

Around 100 people gathered for the light-hearted "breast walk" protest outside police headquarters in Wan Chai district, with some holding up bras and others wearing them over their tops.

"We better watch out as one day police might accuse us of attacking with our penis or buttock," a topless male activist wearing a black bra told the crowd.

Retired teacher James Hon, 66, wearing a pink bra over his white polo shirt, told AFP: "It's the first time to wear a bra in my entire life."

"We have come to this rather odd method to tell the world how ridiculous it is," he said.

The crowd chanted "Breasts are not weapons -- give back our breast freedom" and "Shame on police" as a representative handed in a petition letter to a police officer.

Thirty-year-old Ng Lai-ying was sentenced on Thursday for "assaulting a police officer" during a chaotic protest against mainland Chinese cross-border traders in March.

The clerk was found guilty of using her chest to bump against the arm of chief inspector Chan Ka-po. She was bailed pending an appeal.

"The ruling is absurd. How can breasts be a weapon? We are angry but we also fear that this precedent exploits women's rights to take part in protests," Ng Cheuk-ling, an activist from Hong Kong Women's Coalition on Equal Opportunities, told AFP.

"Police must review their guidelines to handle female protesters," she said.

Ng Lai-ying had previously told the court that she yelled "indecent assault" out of fear immediately after chief inspector Chan's hand landed on her left breast when he failed to grab the strap of her bag, the South China Morning Post reported.

But magistrate Michael Chan Pik-kiu said she "used her female identity to trump up the allegation that the officer had molested" her, calling that a malicious act and harming the officer's reputation, the Post reported earlier.

A massive influx of millions of Chinese tourists to Hong Kong prompted protests early this year by residents who say the visitors have driven up shop rents and prices, leading to clashes with police and arrests.

Hong Kong opened up to Chinese tourists in 2003 in a bid to revive its economy after an outbreak of the respiratory disease SARS, allowing mainland Chinese to visit as individual travellers rather than as part of an organised tour.