Wearing goggles and plastic wrap to protect against police pepper spray, the protesters -- most of them students -- occupied a street outside government headquarters.
They defended their position with metal crowd-control barricades originally brought in by authorities, placing them at both ends of the street.
Earlier in the day, police arrested 74 people who had stormed into a courtyard in the government complex late yesterday, some of them scaling a tall fence.
The standoff follows a week-long strike by students demanding China's Communist leaders allow Hong Kong fully democratic elections in 2017.
Thousands of university and college students who had spent the week boycotting classes were joined yesterday by a smaller group of high school students.
At least 34 people have been injured since the protest began, including four police officers and 11 government staff and guards, authorities said.
One of the officers suffered a gash after being poked by one of the umbrellas the protesters have been using to deflect pepper spray.
Ingrid Sze, a 22-year-old student at the Hong Kong Institute of Education, said she joined the demonstrators after seeing the police take action.
"I didn't participate in the boycott all week. But I saw what was happening to the students live on TV and I thought what the police were doing was so outrageous I had to come out tonight to support the students and my friends," she said.
Hong Kong Secretary for Security Lai Tung-kwok told reporters that police acted appropriately and gave students sufficient warning before starting the process of clearing the square.
But tensions over the Asian financial hub's political future boiled over after China's legislature last month ruled out letting the public nominate candidates, instead insisting they be screened by a committee of Beijing loyalists similar to the one that currently picks the city's leader.