HONG KONG (AP) – Hong Kong police used tear gas on Sunday to try to clear thousands of pro-democracy protesters who had gathered outside government headquarters in a challenge to Beijing over its decision to restrict democratic reforms for the city.

After spending hours holding the protesters at bay, police lobbed canisters of tear gas into the crowd on Sunday evening. The searing fumes sent protesters fleeing down the road, but many came right back to continue their demonstration.

Students and activists have been camped out on the streets outside the government complex all weekend. Students started the rally, but by early Sunday leaders of the broader Occupy Central civil disobedience movement said they were joining them to kick-start a long-threatened mass sit-in to demand an election for Hong Kong's leader without Beijing's interference.

Authorities launched their crackdown after the protest spiraled into an extraordinary scene of chaos as the protesters jammed a busy road and clashed with officers wielding pepper spray.

The protesters were trying tried to reach a mass sit-in being held outside government headquarters to demand Beijing grant genuine democratic reforms to the former British colony.

The demonstrations – which Beijing called ``illegal'' – were a rare scene of disorder in the Asian financial hub, and highlighted authorities' inability to get a grip on the public discontent over Beijing's tightening grip on the city. The protesters reject Beijing's recent decision to restrict voting reforms for the first-ever elections to choose Hong Kong's leader, promised for 2017.

Earlier Sunday, thousands of protesters who tried to join the sit-in breached a police cordon, spilling out onto a busy highway and causing traffic to come to a standstill.

Police officers in a buffer zone manned barricades and doused the protesters with pepper spray carried in backpacks. The demonstrators, who tried at one point to rip apart metal barricades, carried umbrellas to deflect the spray by the police, who were wearing helmets and respirators.

Police had told those involved in what they also call an illegal gathering to leave the scene as soon as possible, warning that otherwise they would begin to clear the area and make arrests.

The use of the tear gas angered the protesters, who chanted ``Shame on C.Y. Leung'' after it was used, referring to the city's deeply unpopular Beijing-backed leader, Leung Chun-ying. To many, it also seemed to mark a major shift for Hong Kong, whose residents have long felt their city stood apart from mainland China thanks to its guaranteed civil liberties and separate legal and financial systems.

Hong Kong ``has changed to a new era so the people have to be awakened. It's no longer the old Hong Kong,'' said one protester, W.T. Chung, 46, who yelled at police officers after they used tear gas. 

Earlier, police said they had arrested 78 people since demonstrations started late Friday, though all but three were released.

Leung said Hong Kong's government was ``resolute in opposing the unlawful occupation'' of the government offices or the financial district by Occupy Central.

``The police are determined to handle the situation appropriately in accordance with the law,'' he said at a news conference.

The Chinese government agency that handles Hong Kong affairs condemned the protests.

``China's central government firmly opposes illegal acts taking place in Hong Kong,'' and fully supports the local government in handling the matter according to the law, the official Xinhua News Agency quoted the Hong Kong and Macau Affairs Office of the State Council as saying.

Protest organizers said police took away several pro-democracy legislators who were among the demonstrators.

Among the protesters was media magnate Jimmy Lai, who owns the popular Apple Daily, Hong Kong's sole pro-democracy newspaper.