It came as new pictures emerged showing the build up of armed forces in the city of Shenzhen, just across the border with Hong Long.
The ambassador, Liu Xiaoming, told a news conference in London that if the situation deteriorates in the region, which is Chinese territory but operates under a different system to the rest of China, it would act to “quell unrest”.
It came after months of anti-China protests which in the last few days have led to the airport, one of the busiest in the world, being closed.
Chinese soldiers walk in formation on the grounds of the Shenzhen Bay Sports Centre in Shenzhen
Mr Xiaoming told reporters: “If the situation deteriorates into uncontrollable unrest, the central government will not sit by and watch, we have enough power to quell the unrest.
“We have enough solutions and enough power within the limits of basic law to quell any unrest swiftly. Their moves are severe and violent offences, and already shows signs of terrorism.
“The central government of China will never allow a few violent offenders to drag Hong Kong down a dangerous road, down a dangerous abyss.”
Pictures released by agencies showed dozens of trucks and armoured personnel carriers and hundreds of troops massing outside the Shenzhen Bay stadium in Shenzhen, bordering Hong Kong in China’s southern Guangdong province.
The stadium is just across a bridge that is one of the main access roads between the mainland and Hong Kong.
Mr Liu also turned his attention to Britain, saying some politicians in the UK still regard Hong Kong as part of the British Empire.
He said: “Good relations between the UK and China have to be based on no UK interference by the UK in China’s internal affairs.”
A look across Shenzhen’s Deep Bay towards Hong Kong on the far right
Pro-Beijing Hong Kong chief executive Carrie Lam had warned earlier that the region was in a “state of… chaos” and could enter “the abyss”.
Sky News reported on Tuesday as protesters brought departures at the international airport to a halt, amid violent skuffles with two people they accused of supporting Beijing.
Mass protests have been taking place across the territory since June, originally against a bill which would have allowed people to be extradited to the mainland.
Chinese military personnel gather inside Shenzhen Bay stadium in Shenzhen, bordering Hong Kong
Some have involved hundreds of thousands of people and many have ended in clashes between the protesters and police.
Mr Liu added that they were extremists masquerading as democracy protesters who were dragging Hong Kong “down a dangerous road”.
“We hope this will end in an orderly way,” he said. “In the meantime we are fully prepared for the worst.”
Earlier, US president Donald Trump suggested trade talks with China could wait until tensions in Hong Kong have eased.
He tweeted: “Of course China wants to make a deal. Let them work humanely with Hong Kong first!”
Praising Chinese President Xi Jinping, calling him a “great leader”, he added: “I have ZERO doubt that if President Xi wants to quickly and humanely solve the Hong Kong problem, he can do it. Personal meeting?”
Mr Liu appeared to respond by saying: “Foreign forces must stop interfering in Hong Kong affairs, stop conniving in violent offences – they should not misjudge the situation and go down the wrong path otherwise they will lift the stone only to drop it on their own feet.
“Evidence shows the situation would not have deteriorated so much had it not been for the interference and incitement of foreign forces.
“Hong Kong is part of China. No foreign country should interfere in Hong Kong’s internal affairs.”
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