More than 100 protesters broke through the gates of Hong Kong's government headquarters as a student demonstration against Beijing's refusal to grant the city unfettered democracy turned angry.

Six people were arrested for offences ranging from forcibly entering government property and public disorder to assaulting an officer, police said in a statement in the early hours of Saturday.

Student groups have been spearheading a civil disobedience campaign along with democracy activists all week in protest at 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.

Around 150 people pushed into the grounds of the complex late Friday, police said, some scaling a high fence, as others outside yelled "open the gates".

"We don't care if we get hurt, we don't care if we get arrested, what we care about is getting real democracy," protester Wong Kai-keung said from the front line of the charge.

Police repeatedly used pepper spray against protesters, who used umbrellas, surgical masks and goggles to protect themselves.

Around 50 demonstrators were still in the complex by early Saturday, surrounded by riot police, who had forcibly removed most of the others.

Around 1,000 protesters had joined the demonstration outside the Southern Chinese city's main government complex through the night.

Numbers had earlier hit more than 2,000 as secondary school pupils, some wearing uniforms, joined university students.

In a statement police said they had arrested six people aged 16 to 29. News footage showed officers taking away a prominent student leader, Joshua Wong.

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.

Teenage pupils had descended Friday on the government headquarters to add their voices to a class boycott kicked off by university students on Monday.

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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. Leung has so far refused to speak to the students or meet their leaders.

- 'Real not fake elections' -

School pupils as young as 13 joined Friday's protest, shouting: "I want real elections not fake ones".

Chung Chun-wai, 17, said many of his friends joined the protest in defiance of their parents, highlighting the often sharp generational divide in the former British colony over its political future.

"I think secondary school students are a part of society and I consider myself a citizen of Hong Kong. That's why I think I need to bear the responsibility to care about society and to voice the real opinions of Hong Kongers," he said.

Meanwhile around 300 people, mostly elderly retirees originally from mainland China, staged a counter-protest close to the site to support Beijing's decision.

Occupy Central, a prominent grassroots pro-democracy group, has vowed to take over the city's Central financial district to push its demand that Hong Kongers be allowed to nominate who can stand for leader.

Occupy co-founder Benny Tai has previously hinted the takeover could begin on October 1, a national holiday when much of the district will be empty.

He told reporters at the city's Foreign Correspondents' Club on Friday: "After next week's action we may not be able to change the standing committee's decision immediately, but if we could have that very strong determination shown, I personally have the confidence that one day democracy will come to Hong Kong."

Last month China said Hong Kongers would be allowed to vote for their leader for the first time in 2017, but that only two or three candidates approved by a pro-Beijing committee could stand.

Britain handed Hong Kong back to China in 1997 under a "one country, two systems" agreement that allows it civil liberties not seen on the mainland, including free speech and the right to protest.