Thousands of pro-democracy protesters massed outside Hong Kong's government headquarters on Saturday night, vowing to keep up an increasingly tense civil disobedience campaign unless Beijing grants more political freedoms.

Defiant demonstrators, almost all wearing convenience-store-bought ponchos and eye protection in anticipation of police using pepper spray, dug in outside the complex where 74 people have been arrested.

Lines of police officers pushed back surges of people with riot shields after earlier clearing dozens who had stormed into the grounds late Friday, as the protests against Beijing's recent announcement it would choose who can stand for Hong Kong chief executive in the 2017 elections turned angry.

During the protest Occupy Central, a prominent grassroots pro-democracy group, announced it was bringing forward its plan to rally thousands of activists and take over key parts of the city's financial district.

The 'takeover' had been expected to begin on October 1, but in a dramatic speech at the protest early Sunday morning, the group's co-founder Benny Tai said: "Occupy Central starts now."

Many in the largely youthful crowd, which had swelled to several thousand with a number of scuffles breaking out with police, also pledged to keep the protest open-ended unless Hong Kong was granted more freedoms.

Organisers played motivational songs and gave speeches that were met with stinging applause. Protesters set up first-aid stations and recycling points. Water, snacks and protection from pepper spray were freely passed out.

"I'm profoundly touched, 99 percent of these people are young people who know what they're doing and know the consequences," Claudia Mo, a legislator representing the pro-democracy Civic Party, told AFP.

"They're not just fighting for democracy, they're fighting for humanity."

Student groups have been spearheading a civil disobedience campaign this week.

"We want to decide our future by ourselves," 23-year-old international relations student Gary Mak told AFP.

"It's a good foundation for all Hong Kong people to fight for democracy."

The police late Saturday urged those gathered in the "unlawful assembly" to leave the area as soon as possible.

More than 2,000 protesters, many of them secondary school pupils and university students, demonstrated Friday, culminating with around 150 demonstrators breaking through police lines into the complex.

Police used pepper spray and dragged many away overnight and into Saturday morning, with the final remainder removed in the afternoon, with arrests for "forcible entry into government premises and unlawful assembly".

- 'Making an impact' -

"This is an amazing turning point," Suki Wong, a recent graduate who works as an accountant, told AFP.

"Hong Kongers usually just lay there and do nothing. This time we're really making an impact."

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.

Protests began on Monday when 13,000 students gathered on a northern campus, according to organisers.

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

Events were supposed to conclude on Friday, but Occupy's announcement suggests the civil disobedience campaign will continue for some time.

The protests come 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.

Political analyst Sonny Lo pointed to a turning point in the city's political development.

"From now on there will be more confrontation, possibly violent ones between citizens and police," he told AFP.

However, Lo said a political deadlock is imminent with Beijing authorities maintaining a hardline stance.

"The government needs to handle the students very carefully, any mishandling will spark larger acts of civil disobedience," he said.

The pro-democracy protesters also drew some support worldwide -- Global Solidarity HK, a coalition of Hong Kong citizens based overseas, organised events in Europe, Canada and America, as well as Taipei.

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.

But tensions have been rising in the southern Chinese city over fears that these freedoms are being eroded.