Hong Kong police have fired repeated volleys of tear gas to disperse pro-democracy protests and baton-charged the crowd blocking a key road in the government district after official warnings against illegal demonstrations.

Student groups are spearheading a civil disobedience campaign along with democracy activists to pressure Beijing into granting full democracy to Hong Kong.

The city's central district descended into chaos on Sunday as chanting protesters converged on police barricades surrounding their colleagues, who had earlier launched a "new era" of civil disobedience.

Riot police staged repeated pepper spray and baton charges and threw tear gas at the crowds. Police have not used tear gas in Hong Kong since 2005.

At least 30 people are being treated for injuries, as students gather at universities around the city in protest over the Chinese government's decision to screen candidates in the 2017 election.

Sorry, this video has expired Sarah Clarke reports on the protests

Organisers said as many as 80,000 people thronged the streets in the city's Admiralty district, galvanised by the arrests of student activists on Friday.

Chanting "remove the blockade" and "shame on you", thousands of protesters blocked some of Hong Kong's busiest streets and milled among the stalled traffic.

About 3,000 protesters blocked a major road in Kowloon, bringing traffic to a standstill and opening up a second significant flash point in the key financial hub.

Police have warned they could use greater force if the tens of thousands demonstrators do not leave the area.

Commissioner Andy Tsang is standing by his officers' actions.

"We only use force when it's necessary and ensure only the minimum necessary force is used," he said.

"The police have a duty to protect the members of public and will take action as necessary to ensure this is done."

Earlier, Hong Kong leader Leung Chun-ying had pledged "resolute" and "lawful" action against the movement known as Occupy Central with Love and Peace.

"The police are determined to handle the situation appropriately in accordance with the law," Mr Leung said just hours before the charge began.

"We've been encouraging people to express their views in a rational and peaceful manner and accommodate different views. The Hong Kong government rationally is against the Occupy Central Movement and the founders' move to occupy central illegally."

A spokesperson for China's Hong Kong and Macau Affairs Office added that the central government fully supported Hong Kong's handling of the situation "in accordance with the law".

An estimated 80,000 pro-democracy protesters have blocked streets in central Hong Kong. ( AFP/Xaume Olleros )

'We will win this war with love and peace'

Hong Kong returned to Chinese rule in 1997 under a formula known as "one country, two systems" that guaranteed a high degree of autonomy and freedoms not enjoyed in mainland China.

But Beijing last month rejected demands for people to freely choose the city's next leader, prompting threats from activists to shut down Central. China wants to limit elections to a handful of candidates loyal to Beijing.

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While promising a fresh round of public consultation, Mr Leung also described Beijing's decision as "legally binding" but said the government will do its best to address the protesters' concerns.

"Let me make an appeal to the very sectors of the community to engage in rational discussions through peaceful and lawful means so as to allow the 5 million eligible voters in Hong Kong to elect the chief executive in 2017 for the first time in Hong Kong's history by one person, one vote," he said.

Publishing tycoon Jimmy Lai, a key backer of the democratic movement, said he wanted as big a crowd of protesters as possible to thwart any crackdown.

"The more Hong Kong citizens come, the more unlikely the police can clear up the place," Mr Lai said.

"Even if we get beaten up, we cannot fight back. We will win this war with love and peace."

No independent estimate of the crowd numbers was available but the action is being seen as the most tenacious civil disobedience action since 1997.

A week of protests escalated into violence when demonstrators broke through a cordon late on Friday and scaled a fence to invade the city's main government compound.

Police used pepper spray to disperse the crowd, arresting 78 people.

ABC/Reuters