China has expressed its strong displeasure over a meeting between Hong Kong pro-democracy campaigners and US Vice-president Joseph Biden at the White House.
The country urged Washington to distance itself from their internal affairs and cautioned against interference in Hong Kong politics.
The two pro-democracy supporters, Anson Chan and Martin Lee, went to Washington to lobby for international support against Beijing's growing political domination in Hong Kong.
They also met US Democratic Party leader Nancy Pelosi in the House of Representatives to brief her about what they see as China's indirect political autocracy in Hong Kong, according to Reuters.
The former British colony became a Special Administrative Region of China in 1997 with the rider that it would retain its civil liberties.
Hong Kong was long accustomed to broad freedoms taken for granted in a capitalist economic setup and society.
Beijing had promised elections for Hong Kong's chief executive in 2017, but democracy activists fear that China will install its own candidate for the position, as China ruled out any possibility of Hong Kong voters being allowed to decide on the nominees.
China's refusal to let people choose candidates for the democratically elected office triggers frequent protests in Hong Kong.
Backing the meeting, the White House reiterated its stance on political freedom, saying that Biden had "underscored our long-standing support for democracy in Hong Kong".
"The future of freedom and democracy in Hong Kong is under serious threat," US Senator Sherrod Brown, said in a statement.
"China is already placing 'pre-conditions' on who can run [in 2017], raising serious doubts about whether the elections will be free and fair."
However, China's foreign ministry spokesman said that Hong Kong was at a critical juncture of political reforms, and interference by any country in the region will not be tolerated, the state-run Xinhua news agency reported.
A foreign ministry official was quoted as saying that the US must "proceed discreetly" on Hong Kong affairs, and "refrain from interfering."
Activists are reportedly planning to lead a civil disobedience moment called "Occupy Central" in Hong Kong's central business district if Beijing does not agree to hold democratic elections in accordance with international norms.