China's army in Hong Kong has released a three-minute video showing troops carrying out "anti-riot" exercises, in what is being seen as a thinly veiled warning to pro-democracy protesters.
The video, posted to social media, begins with a soldier shouting: "All consequences are at your own risk."
Tensions are high in Hong Kong after weeks of anti-government protests.
On Wednesday, more than 40 activists appeared in court charged with rioting, after Sunday's protest turned violent.
If convicted, they could face up to 10 years in prison.
Hong Kong, a former British colony, is part of China but enjoys unique freedoms not seen on the mainland.
China's People's Liberation Army (PLA) has so far stayed out of the protests that have gone on for eight consecutive weekends and brought parts of central Hong Kong to a standstill - leaving the territory's police to deal with the unrest.
But at a reception to mark the 92nd anniversary of the PLA on Wednesday, the commander of the Chinese army's garrison in Hong Kong said the protests had "seriously threatened the life and safety of Hong Kong citizens, and violated the bottom line of 'one country, two systems'.
"This should not be tolerated and we express our strong condemnation," Chen Daoxiang said, in comments reported by the South China Morning Post, a Hong Kong newspaper.
PLA #HongKong garrison shows its commitment to safeguarding the country's sovereignty and security, as well as Hong Kong's prosperity and stability through a video released on Wednesday. pic.twitter.com/GlOI9K56Yw— China News 中国新闻网 (@Echinanews) August 1, 2019
The army video, posted to the Chinese social media site Weibo, shows lines of troops carrying shields and batons performing anti-riot drills. It showcases tanks, rocket launchers, water cannon and barbed wire before featuring heavily armed troops descending from helicopters and shooting their way through the streets and into people's homes.
Protesters can also be seen being arrested and walked, with their arms tied behind their backs, to "detention points".
Observers believe the video was likely to have been filmed in Hong Kong because the local Cantonese dialect is spoken, and it features a Hong Kong taxi and a flag almost identical to one used by Hong Kong police.
The BBC's Celia Hatton, in Beijing, says earlier posts from the Chinese garrison in Hong Kong showed patriotic images of smiling soldiers. The increasingly aggressive posturing featured in the latest video will fuel concerns that China could eventually use military force to end violent protests.
But the Chinese government is still refusing to answer such concerns directly, our correspondent notes. When asked about the video, the Chinese foreign ministry in Beijing said only that the military would be able to interpret its message.
The demonstrations in Hong Kong began in March over a controversial bill that would have enabled extraditions to mainland China.
The government has since suspended the bill, but protesters want it withdrawn completely.
The demonstrations have also broadened into a wider movement, with activists demanding democratic reform and an independent inquiry into police violence.
Violent clashes erupted on Sunday as police fired tear gas and rubber bullets at protesters.
In a rare intervention on Monday, China's top policy office in Hong Kong condemned the "horrendous incidents" that have caused "serious damage to the rule of law".
A spokeswoman for the Hong Kong and Macau Affairs Office said the territory's "top priority" was to "restore social order".