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.

“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”.

Meanwhile, thousands of Chinese military personnel waving red flags paraded at a sports stadium in a city across the border from Hong Kong on Thursday, an AFP reporter witnessed.

Armoured vehicles were also seen inside the stadium in Shenzhen, with the event taking place as concerns build that China may intervene to end 10 weeks of unrest.

State-run media reported this week that the elements of the People’s Armed Police (PAP), under the command of the Central Military Commission, were assembling in Shenzhen.

Some of the personnel inside the stadium on Thursday had armed police insignias on their camouflage fatigues, according to the reporter.

The security forces could be seen moving in formation inside the stadium, and occasionally running, while others rode around outside on motorbikes.

Outside the stadium – which is around 7km from Hong Kong – there were also dozens of trucks and armoured personnel carriers.

“I don’t know why they’re here, but it could be related to Hong Kong,” a ticket vendor at the stadium said.

The People’s Daily and Global Times, two of the most powerful state-run media outlets, published videos on Monday of what it said was the PAP assembling in Shenzhen.

The Global Times editor-in-chief, Hu Xijin, said the military presence in Shenzhen was a sign that China was prepared to intervene in Hong Kong.

“If they do not pull back from the cliff and continue to push the situation further beyond the critical point, the power of the state may come to Hong Kong at any time,” Hu wrote.

US President Donald Trump also said on Tuesday American intelligence had confirmed Chinese troop movements toward the Hong Kong border.

“I hope it works out for everybody including China. I hope it works out peacefully, nobody gets hurt, nobody gets killed,” Trump said.

The protests have become increasingly violent and led to Hong Kong’s airport being paralysed for two days this week.

The Chinese military has not interfered in Hong Kong since the handover but it can should it be called on by the city’s government to maintain “public order”.

James Char, a military expert at Singapore’s Nanyang Technological University, said the deployment to Shenzhen was both to project an image of domestic strength as well as “calculated message to the protesters to think twice about growing or continuing with their recent intensified demonstrations”.

“We can be certain the regime understands that sending in troops to Hong Kong will inflame the protests and the protesters’ anti-China grievances,” he said.