Hong Kong Protests: The movement represents the greatest challenge to China's authority (AFP)

China will not "sit by and watch" and is ready to "quell the unrest swiftly" if the crisis in Hong Kong becomes "uncontrollable", China's ambassador to London said on Thursday.

"If the situation deteriorates further into unrest uncontrollable by the SAR (Special Administrative Region) government, then the central government will not sit by and watch," Liu Xiaoming said in a televised press conference.

"We have enough solutions and enough power to quell the unrest swiftly," he said.

Images taken by AFP earlier on Thursday showed thousands of Chinese military personnel waving red flags and parading at a sports stadium in the city of Shenzhen, just across the border from Hong Kong.

Dozens of armoured personnel carriers and supply trucks were also parked nearby.

"We hope this will end in an orderly way. In the meantime we are fully prepared for the worst," Liu said.

He also protested against "foreign interference" in the Hong Kong protests and urged British Prime Minister Boris Johnson's government to handle the issue with "great caution".

"I think some politicians in this country... still regard Hong Kong as part of the British empire," he said.

The Hong Kong protests were sparked by opposition to a plan to allow extraditions to the mainland, but have since morphed into a wider -- sometimes violent -- call for democratic rights.

The movement represents the greatest challenge to Beijing's authority since the city was handed back by the British in 1997 under a deal that allowed it to keep freedoms that many Hong Kongers feel are being eroded.

China earlier this month warned Britain to stop "meddling" after Foreign Secretary Dominic Raab called Hong Kong leader Carrie Lam and emphasised the need for a "fully independent investigation into recent events".