Chinese President Xi Jinping has said there "is no reason to change" the One Country, Two Systems arrangement under which Hong Kong is governed, but insisted that the city "must be administered only by patriots".
Critics of China say that system has already been fundamentally undermined by Beijing's imposition of a sweeping national security law, under which scores of pro-democracy politicians and campaigners have been arrested.
The UK Foreign Secretary Liz Truss said that, because of the law, "authorities have stifled opposition, criminalised dissent and driven out anyone who can speak truth to power".
Mr Xi visited Hong Kong as part of the celebrations to mark 25 years since the UK handed over the territory to China and to swear in the city's new leader, John Lee.
Mr Lee is a former security chief who played a key role in putting down the pro-democracy protests of 2019 and has been sanctioned by the United States.
In his first speech as chief executive, Mr Lee said that Hong Kong had successfully dealt with "foreign forces' interference in Hong Kong's internal affairs that threatened the national security of the country".
Lord Patten, the last colonial governor of Hong Kong, told Sky News that China had not kept to the commitments it made ahead of handover.
"They catastrophically and comprehensively broke their word about Hong Kong, which was set out in a treaty lodged at the United Nations, which guaranteed Hong Kong its continuing way of life until 2047, and also promised that China would develop democracy in Hong Kong.
"Well, fat chance of that happening with the secret police taking over."