HONG KONG • As Hong Kong's political crisis simmers amid heated protests, China's People's Liberation Army (PLA) in Hong Kong released a video showing footage of "anti-riot" exercises, and its top brass warned that violence is "absolutely impermissible".

The three-minute video posted on the Hong Kong garrison's official Weibo social media account late on Wednesday included footage of troops firing guns and rockets, and of light tanks, attack helicopters and missile launchers.

PLA troops have remained in their barracks since protests started in April, leaving Hong Kong's police force to deal with the massive and often violent protests in the Asian financial hub.

On Wednesday, 44 people were charged with rioting, the first time the authorities have resorted to using the charge.

In one sequence in the PLA video, titled "Anti-riot drill footage", ranks of troops advance with shields and truncheons on "pro-testers", firing rifles into the air.

Tear gas and water cannon rain down on protesters, as armoured personnel carriers with battering rams race forward, and troops lay barbed-wire coils on the ground. "All consequences are at your own risk," one soldier shouts in Cantonese, the main Chinese dialect in Hong Kong, during the video clip.

A red flag with the words "Warning. Stop charging or we use force" is also held aloft, similar to what Hong Kong police have long used during protests.

At the end of the clip, several protesters with their hands cuffed can be seen being taken away.

Hong Kong has been plunged into its biggest political crisis since the former British colony's return to Chinese rule in 1997. There have been waves of protests against a now-suspended extradition Bill which would see people sent to mainland China for trial in Communist Party-controlled courts.

The protests, which started in April, have now transformed into broader demands, including the resignation of Hong Kong's embattled leader Carrie Lam, and calls for a "free Hong Kong".

As tensions continue to rise in Hong Kong as clashes between protesters and police grow violent, the Chinese authorities have strongly condemned the violence and issued reminders that the Hong Kong authorities can request the assistance of the PLA Hong Kong garrison if necessary.

PLA spokesman Wu Qian referenced the Garrison Law at a briefing last week, noting that Article 14 has "clear stipulations". Should Beijing approve, the garrison would send troops to carry out the specified tasks.

Diplomats and foreign security analysts are watching the situation closely, but believe there is little appetite in Beijing for the PLA to be deployed on the streets of Hong Kong.

The role of the PLA in Hong Kong has long been one of the most sensitive elements of the city's handover to China.

"We believe that the Hong Kong garrison of the People's Liberation Army will continue to become a stabilising pillar for Hong Kong's long-term prosperity and stability," Chinese Foreign Ministry spokes-man Hua Chunying said at a briefing in Beijing when asked about the video.

Amid growing speculation that troops could be unleashed to quell violence, police chiefs have insisted that their forces are capable of maintaining order.

Hong Kong returned to Chinese rule with a guarantee of its freedoms, including the right to protest and an independent judiciary not enjoyed on the mainland, for at least 50 years.

Under its mini-Constitution, the Basic Law, defence and foreign affairs are managed by Communist Party leaders in Beijing.

PLA's Hong Kong garrison commander Chen Daoxiang said on Wednesday that the violent protests in the territory are "absolutely impermissible", according to the state-run China Daily newspaper.

Speaking at a reception to mark the 92nd anniversary of the PLA, Major-General Chen said the protests were "putting the bottom line of the 'one country, two systems' principle to the test".

He added that the PLA would "resolutely safeguard" the country's sovereignty and security, as well as Hong Kong's stability.