More than 50 protesters held out at Hong Kong government headquarters Saturday after storming the complex overnight as a week-long protest against Beijing's refusal to grant the city unfettered democracy turned angry.

Riot police using pepper spray cleared out more than 100 demonstrators in the early hours of Saturday, dragging many away and arresting 13, in the tensest scenes yet in a recent series of protests.

Student groups have been spearheading a civil disobedience campaign this week in response to Beijing's announcement last month that it would choose who can stand for Hong Kong's top post of chief executive in elections in 2017.

More than 2,000 protesters, many of them secondary school pupils and university students took part in protests on Friday at the city's main government headquarters, culminating with around 150 demonstrators breaking through police lines to occupy its forecourt late on Friday night.

By Saturday morning, police pushed hundreds of protesters away from the site into a nearby street, liberally using pepper spray, an AFP correspondent at the scene said.

"The use of pepper spray is disgusting," Mia Chan, an accountant, said. "When you put on that uniform, you have a moral duty to protect," Chan said.

Police have said the 13 arrested were aged between 16 and 35 and detained for forcible entry into government premises, disorderly conduct in public place and assaulting a police officer.

Among those arrested was a prominent student leader, 17-year-old Joshua Wong.

'Hope lies with the people'

Protesters who remained in the complex Saturday morning hoisted a sign saying "Hope lies with the people, change starts with resistance," written in black letters on white cloth.

"This is an amazing turning point. Hong Kongers usually just lay there and do nothing, this time, we're really making an impact," Suki Wong, a recent graduate who works as an accountant, told AFP.

Protesters used umbrellas to protect themselves from being pepper-sprayed by police wearing riot gear including helmets and body-length plastic shields.

"We're disappointed because the police just treated us like an object, like a roadblock that has to be moved," 20-year-old Lu Yiu, who spent nine hours caught between police lines unable to use the toilet or sleep.

"Everyone was crying as we were pushed onto the street from the extreme use of force," Lu said, adding that his throat and nose burned from inhaling pepper spray.

In a statement, the government "expressed regret" that protesters had stormed the complex, saying security personnel, police officers and protesters had suffered injuries but without giving details.

Friday's action was supposed to be the culmination of a week of protests that began on Monday when 13,000 students gathered on a campus in the north of the city, according to organisers.

On Thursday night, more than 2,000 people took their protest to the residence of Hong Kong leader Leung Chun-ying with the hope of speaking to him. However, Leung has so far refused to speak to the students or meet their leaders.

The series of protests comes after China last month said Hong Kongers would be allowed to vote for their leader for the first time in 2017, but that only candidates approved by a pro-Beijing committee could stand.

A protest in July saw half-a-million-people -- according to organisers -- take to the streets to express their discontent at what they see as China's increasingly tight grip on the city.

Handed back to China by former colonial ruler Britain in 1997, Hong Kong is governed under a "one country, two systems" agreement that grants civil liberties not seen on the mainland, including free speech and the right to protest.