China has launched one of its fiercest assaults on Hong Kong's treasured autonomy with its move to impose a security law, pro-democracy campaigners said on Friday as they vowed to take to the streets in protest.

The proposal for the security law - expected to ban treason, subversion and sedition - was introduced into China's rubber-stamp parliament at the opening of its annual session on Friday morning.

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It followed repeated warnings from China's communist leaders they would no longer tolerate dissent in Hong Kong, a semi-autonomous city that endured seven months of massive pro-democracy protests last year.

"This is the end of Hong Kong, this is the end of 'One Country, Two Systems', make no mistake about it," Civic Party lawmaker Dennis Kwok told reporters.

Prominent democracy activist Joshua Wong said China's message to protesters seeking to maintain their freedoms was clear.

"Beijing is attempting to silence Hongkongers' critical voices with force and fear," Wong said, while expressing defiance.

"HKers will not scare off in the face of wolf warrior policy."

Limited form of autonomy

On internet threads and chat apps used by the pro-democracy movement, there were calls to resume the protests of last year that were largely subdued in recent months because of coronavirus restrictions.

"Hong Kong people will have to face the choice for their future," said one comment on the Telegram platform often used by protesters, calling for people to rally on Sunday.

Hong Kong has been allowed a limited form of autonomy since returning from British to Chinese rule in 1997, with those unique freedoms enshrined under a "One Country, Two Systems" handover agreement.

However, a huge pro-democracy movement has built in the face of fears China has been steadily eroding those freedoms.

Activists said the planned national security law, if implemented, would be one of the most significant erosions of Hong Kong's liberties since 1997.

Local pro-democracy lawmaker Tanya Chan said Beijing had "shown zero respect for Hong Kong people".

"Many Hong Kongers must be as angry as us now, but we must remember not to give up," she added.

In announcing the plans late on Thursday night, the Communist Party gave few specifics for the planned law other than it would strengthen "enforcement mechanisms" for Hong Kong.

Article 23 of Hong Kong's mini-constitution, the Basic Law, says the city must enact a law to prohibit "treason, secession, sedition (and) subversion" against the Chinese government.

But the clause has never been implemented due to fears it would destroy Hong Kong's cherished civil rights.

An attempt to have Article 23 pass through Hong Kong's legislature in 2003 was shelved after half a million people took to the streets in protest against it.

China's move this week would circumvent Hong Kong's legislature by having it imposed by the national parliament, according to Wong and other campaigners.

The United States reacted swiftly to China's announcement, with State Department spokesperson Morgan Ortagus warning that imposing such a law on Hong Kong would be "highly destabilising, and would be met with strong condemnation from the United States and the international community".

US President Donald Trump gave a vague initial response that nevertheless warned of a stronger reaction.

"I don't know what it is, because nobody knows yet. If it happens, we'll address that issue very strongly," Trump said.

The US Congress late last year angered China by passing a law that would strip Hong Kong's preferential trading status if it is no longer considered autonomous from the mainland.

The State Department warned China's actions could impact its decision on that status.