HONG KONG police have used tear gas, pepper spray and high-pressure water hoses against protesters who have massed outside government buildings.

Thousands of protesters blocked entry to the government headquarters, delaying a legislative session on a proposed extradition bill that has become a lightning rod for concerns over greater Chinese control and erosion of civil liberties in the territory.

The overwhelmingly young crowd of demonstrators filled nearby streets, overturned barriers and tussled with police outside the offices where the bill, which would allow criminal suspects in Hong Kong to be sent for trial in mainland China, was due to be discussed.

HeraldScotland:

A government statement said the session of Hong Kong's Legislative Council that was scheduled to begin at 11am local time on Wednesday had been "changed to a later time" yet to be decided.

A statement from a Hong Kong administrator earlier urged protesters "to stay calm and leave the scene as soon as possible and not to commit any crime".

The delay appeared to be at least a temporary victory for the bill's opponents.

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The protests have prompted Hong Kong's biggest political crisis since pro-democracy demonstrations closed down parts of the city centre for more than three months in 2014.

They pose a challenge to China's ruling Communist Party and President Xi Jinping, who has in the past said he would not tolerate Hong Kong being used as a base to challenge the party's authority. But they are also giving vent to young Hong Kongers alienated by a political process dominated by the territory's economic elite.

Protesters said they hope the blockade will persuade chief executive Carrie Lam's administration to shelve the proposed amendments to the Fugitive Offenders Ordinance and the Mutual Legal Assistance in Criminal Matters Ordinance.

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One protester, who gave only his first name Marco, said: "We want the government to just set the legislation aside and not bring it back."

A fellow protester who gave her name as King said the protest is a watershed moment for Hong Kong's young generation, who face difficult job prospects and skyrocketing housing prices.

"We have to stand up for our rights or they will be taken away," she said.

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The reluctance of protesters to be identified by their full names and professions - many wore surgical masks to obscure their facial features - reflects an increasingly hard-line approach to civil unrest by the authorities.

Such actions are never tolerated in mainland China and Hong Kong residents can face travel bans and other repercussions if they cross the border.

Protesters also appear mindful of Beijing's growing use of electronic surveillance such as facial recognition technology to build dossiers on those it considers politically unreliable.

Another statement from the government's information office said access roads leading to the Central Government Offices were blocked and police had implemented traffic arrangements.

Staff members were advised not to go to into work and those already on the premises were told to "stay at their working place until further notice".

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Under its "one country, two systems" framework, Hong Kong was supposed to be guaranteed the right to retain its own social, legal and political systems for 50 years following its handover from British rule in 1997.

However, China's ruling Communist Party has been seen as increasingly reneging on that agreement by forcing through unpopular legal changes.

A crowd began gathering outside the Legislative Council on Tuesday night, and the US Consulate warned people to avoid the area, exercise caution and keep a low profile.

The legislation has become a lightning rod for concerns about Beijing's increasing control over the semi-autonomous territory.

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Ms Lam has consistently defended the legislation as necessary to close legal loopholes with other countries and territories. A vote is scheduled for June 20.

Critics believe the extradition legislation would put Hong Kong residents at risk of being entrapped in China's judicial system, in which opponents of Communist Party rule have been charged with economic crimes or ill-defined national security offences, and would not be guaranteed free trials.

Ms Lam, who cancelled her regular question and answer session on Wednesday, said the government had considered concerns from the private sector and altered the bill to improve human rights safeguards. She said without the changes, Hong Kong would risk becoming a haven for fugitives.

She emphasised extradition cases would be decided by Hong Kong courts.

Hong Kong currently limits extraditions to jurisdictions with which it has existing agreements and to others on an individual basis. China has been excluded from those agreements because of concerns over its judicial independence and human rights record.