Nearly 200 political figures from around the world have denounced Beijing's proposed national security laws for Hong Kong.
Earlier this week, China's ruling Communist Party announced it wants to clamp down on dissent in Hong Kong by introducing a national security law.
Pro-democracy activists say the move to ban "treason, secession, sedition and subversion" will destroy Hong Kong’s autonomy.
In a joint statement organised by former Hong Kong Governor Christopher Patten and former British Foreign Secretary Malcolm Rifkind, 186 law and policy leaders said the proposed laws are a "comprehensive assault on the city's autonomy, rule of law and fundamental freedoms" and a "flagrant breach" of the Sino-British Joint Declaration that returned Hong Kong to China in 1997.
"If the international community cannot trust Beijing to keep its word when it comes to Hong Kong, people will be reluctant to take its word on other matters," they wrote.
US officials said the Chinese legislation would be bad for both Hong Kong's and China's economies and could jeopardise the territory's special status in US law.
China has dismissed other countries' complaints as meddling.
Some of Trump's fellow Republicans - Senator Marco Rubio, acting chair of the Intelligence Committee, and Senator Ted Cruz - signed the statement.
Democratic signatories included Representative Eliot Engel, head of the House Foreign Affairs Committee, and Adam Schiff, chairman of the US House Permanent Select Committee on Intelligence.
In the UK, 44 members of the House of Commons and eight members of the House of Lords also signed the statement, alongside figures from across Europe, Asia, Australia and North America.
It follows a joint statement from Foreign Secretary Dominic Raab, Australian foreign affairs minister Marise Payne and Canadian foreign minister Francois-Philippe Champagne on Friday, which said: “We are deeply concerned at proposals for introducing legislation related to national security in Hong Kong.
“The legally-binding joint declaration, signed by China and the UK, sets out that Hong Kong will have a high degree of autonomy.
“It also provides that rights and freedoms, including those of the person, of the press, of assembly, of association and others, will be ensured by law in Hong Kong, and that the provisions of the two UN covenants on human rights (the International Covenant on Civil and Political Rights and International Covenant on Economic, Social and Cultural Rights) shall remain in force.
“Making such a law on Hong Kong’s behalf without the direct participation of its people, legislature or judiciary would clearly undermine the principle of ‘one country, two systems’, under which Hong Kong is guaranteed a high degree of autonomy.”
Downing Street has also urged China to respect Hong Kong’s rights amid concerns Beijing is tightening its grip on the city.
Boris Johnson’s official spokesman said: “We are monitoring the situation closely. We expect China to respect Hong Kong’s rights and freedoms and high degree of autonomy.
“As a party to the joint declaration, the UK is committed to upholding Hong Kong’s autonomy and respecting the one country two systems model.”
Asked if the proposed law met the one country two system principle, he replied: “We are monitoring this closely and our immediate priority is to clarify the details of what is being suggested.”