Around 150 people have poured onto the streets for the "breast walk" protest outside police headquarters in Wan Chai district, wearing bras over their tops.
"We better watch out as one day police might accuse us of attacking with our penises or butts," a topless male activist with a bra on told the crowd.
The crowd shouted out "Breasts are not weapons —give back our breast freedom" while blaming the police for their actions.
The woman was found guilty of pressing her breast against a cop, thereby "assaulting a police officer", during a dispute which pitted locals from Hong Kong against Chinese mainlanders.
"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 the Hong Kong Women's Coalition on Equal Opportunities, told AFP.
She claimed the police officer intended to stop her from grabbing her bag but instead his hand landed on her breast and she accused him of indecent assault, whereas he accused her of assaulting him with her breast.
The Tuen Mun court held that the woman had tried to damage the policeman's reputation and sentenced her in prison for 105 days.
She had used "her female identity to trump up the allegation that the officer had molested" her, the South China Morning Post quoted him as saying.
A massive inrush of Chinese tourists to Hong Kong led to collisions with police early this year, when locals complained that visitors had caused their shop rents and prices to grow.