Hong Kong is sliding towards becoming a police state, US senator Josh Hawley warned Monday, as tens of thousands filled the strife-torn city's streets waving American flags and calling on Washington to punish China over sliding freedoms.

The international finance hub was battered by another weekend of unrest as hardcore pro-democracy protesters and police fought running battles with officers warning the violence had now reached "life-threatening levels". rotests pushing for greater democratic freedoms and police accountability have raged for the last 19 weeks and there is little end in sight as Beijing and local leaders refuse concessions.

On Monday night a huge crowd filled Hong Kong's commercial district to show support for a US bill that could dramatically alter Washington's relationship with the trading hub.

The Hong Kong Human Rights and Democracy Act, which could be discussed and voted on by the House of Representatives as early as this week, would require annual reviews of the territory's special trading status -- and potentially sanction some Chinese officials.

Monday's rally was the largest in weeks partly because it was granted permission to go ahead by police.

"We are here to make an urgent call out to the international community to support us, we have no other way," a 24-year-old art student, who gave her surname as Chun, told AFP.

Among the bill's sponsors is Hawley, a Republican senator for Missouri who was in Hong Kong on a two-day trip during which he watched protests on Sunday.

"The situation here is urgent," he told reporters shortly before flying back to Washington before Monday night's rally.

Asked what his message would be on returning to Congress he replied: "That Hong Kong is in danger of sliding towards a police state and that representative government in Hong Kong is at risk, and that the one country two systems model is at risk."

One country, two systems is the deal China agreed to ahead of the 1997 handover by Britain allowing Hong Kong to keep its unique freedoms such as free speech and an independent judiciary for 50 years.

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Democracy activists inside Hong Kong have long accused Beijing of chipping away at those freedoms, a gripe that has fuelled years of growing resentment which exploded this summer.

- Threats from Xi -

China has accused "external forces" of fuelling unrest in the semi-autonomous city and has seized on supportive comments by western politicians to back its claims.

"Anyone who attempts to split any region from China will perish, with their bodies smashed and bones ground to powder," Chinese president Xi Jinping said Sunday.

Hawley, at 39 the youngest serving senator and a vocal China hawk, described Xi's comments as "violent rhetoric" that illustrated why he believed the US and its European allies needed to take a stronger line at Beijing's growing regional threat.

He also said Hong Kong protesters should eschew violence and not "mimic the behaviour of Beijing and (its) supporters".

Republican senators such as Marco Rubio and Ted Cruz have become some of the most vocal supporters of Hong Kong's democracy movement on Capitol Hill -- although the proposed bill is rapidly gathering bipartisan support, especially in the Senate.

Cruz also made a visit to Hong Kong on Saturday during which he accused Beijing of trying to impose a "dictatorship" on the city.

At a briefing on Monday, police said they were facing increasingly dangerous attacks from protesters, including the first use of a crude home made explosive device and a policeman who was slashed in the neck with a knife.

"Violence against police has reached a life-threatening level," Deputy Commissioner Tang Ping-keung said.

But police have also been accused of using excessive force throughout the summer.

In the latest allegation, a driver for local news station NowTV accused officers of assaulting him in custody after he was briefly detained.