Hong Kong police fire tear gas at pro-democracy protesters during the first day of the mass civil disobedience campaign Occupy Central.

Police repeatedly fired tear gas after tens of thousands of pro-democracy demonstrators brought parts of central Hong Kong to a standstill yesterday, a dramatic escalation of protests that have gripped the city for days.

There were chaotic scenes, with protesters screaming “Shame!” at police as they tried to shield themselves from the clouds of gas, said AFP reporters on the scene.

The tear gas volleys incensed the predominantly youthful crowds demonstrating against Beijing’s refusal to grant full democracy to the semi-autonomous city, as a tense week of largely contained student-led demonstrations exploded into mass angry street protests.

Several scuffles broke out between police in helmets, gas masks and riot gear and demonstrators angered by the tear gas, which was last used in Hong Kong in 2005.

“We are unarmed, just standing here, there was no warning for the gas,” Harry Hung told AFP.

As the night wore on protesters were locked in a tense standoffs with police, some of whom had batons drawn and rubber bullet rifles slung over their shoulders.

“This is unbelievable. This is a peaceful protest and the police are the ones using violence,” said demonstrator Jade Wong. “The level of police violence here is just like mainland China, it was never like this before.”

Renewed rounds of tear gas were fired shortly before 10pm (1400 GMT) forcing demonstrators to run from the choking clouds, an AFP reporter saw.

But it did little to stem the tide of demonstrators occupying more than 800m of a vital multi-lane highway usually filled with whizzing vehicles.

Officers had earlier used hand-held pepper spray on demonstrators who had spilt onto the highway after breaking through barricades set up to stop people joining the crowds camped outside Hong Kong’s government headquarters since Friday.

Traffic ground to a halt, with crowded commuter busses abandoned on the spot as demonstrators cheered and embraced each other in the middle of the road.

China, which stations a military garrison in Hong Kong, said it was confident the city’s administration could handle the protest.

Beijing “firmly opposes all illegal activities that could undermine rule of law and jeopardise ‘social tranquillity’ and it offers its strong backing” to the Hong Kong government, a spokesman for China’s Hong Kong and Macau affairs office was quoted by the Xinhua news agency as saying.

Last month Beijing said that it would allow elections for Hong Kong’s leader in 2017, but would insist on vetting the candidates - something demonstrators have decried as “fake democracy”.

Students have boycotted classes in the past week, with protesters also mobbing the city leader’s residence and storming into the complex housing government headquarters.

Prominent pro-democracy group Occupy Central yesterday threw its weight behind the protests, saying they were bringing forward a mass civil disobedience campaign that had been due to start on October 1.

“Occupy Central starts now,” co-founder Benny Tai told the crowds outside government headquarters.

The group had sparked months of heated debate in the city of 7mn over its plan to bring Hong Kong’s financial district to a standstill with a mass sit-in. Yesterday, they appeared to have come close to reaching that goal.

Hong Kong’s leader, Chief Executive Leung Chun-ying, told a press conference his administration was “resolute in opposing the unlawful occupation actions by Occupy Central”.

He said his government would hold more public consultations on the planned political changes - a move already scheduled before the protests.

Ryan Chung, a 19-year old student watching events unfold, said: “We have the right to stay here and to protest. The world needs to know what is happening in Hong Kong. They need to know we want democracy but don’t have it.”

Political analyst Sonny Lo said the protests marked a turning point in the city’s long campaign for democracy.

“From now on there will be more confrontation, possibly violent ones between citizens and police,” he said.

But he added that with Beijing maintaining a hardline stance, it was difficult to see a way out of the standoff.

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

Robert Law, a 30-year-old advertising executive, accused the police of heavy handed tactics.

“We’re simply using our freedom of speech and they’re treating us like rioters,” he said.

Former colonial power Britain handed Hong Kong back to China in 1997 under a “one country, two systems” deal that guarantees liberties not seen on the mainland, including freedom of speech and the right to protest.

But tensions have been growing over fears that these freedoms are being eroded, as well as perceived political interference from Beijing.