Amnesty International accused the police of "failing in their duty" to protect demonstrators on Friday evening, saying officers "stood by and did nothing" when counter demonstrators and suspected triad members clashed with activists at protest sites in Mongkok and Causeway Bay.
"Women and girls were among those targeted, including incidents of sexual assault, harassment and intimidation" in the commercial hubs, Amnesty said in a statement.
The pro-democracy demonstrations have taken over major thoroughfares in the city, causing traffic to grind to a standstill for the past week.
An AFP reporter spoke to one young woman protester in Causeway Bay who said her three friends had been assaulted by a man who was opposed to the Occupy movement, whose supporters have vowed to stay on the streets until their goal of universal suffrage is achieved. The three women were all crying as they were bundled into a police van.
"We are making further inquiries -- these girls say they were indecently assaulted," a police officer on the scene told AFP.
Tensions remained high throughout Saturday at the three main protest sites where democracy activists have held sit-ins for the last week as organisers reinforced barricades and set up lookout points in case of further assaults by counter demonstrators.
An AFP reporter in Mongkok on Saturday afternoon heard a female counter-demonstrator tell pro-democracy crowds through a loudspeaker: "Women are supposed to be touched by men." She spoke in Cantonese with a mainland accent.
Amnesty said that a woman at the Mongkok clashes had also been attacked on Friday.
"A man grabbed her breasts while she was standing with other protesters at around 4:00 pm (0800 GMT)," they said in a statement. "She also witnessed the same man assault two other women by touching their groins," it added, with other protesters intervening to help her.