HONG KONG (Reuters) - China has cautioned the United States not to interfere in Hong Kong affairs after Vice President Joseph Biden met two prominent pro-democracy advocates who have warned of Beijing's tightening control of the territory, state news agency Xinhua said.

A former British colony that reverted to Chinese rule in 1997, Hong Kong enjoys considerable autonomy and broad freedoms as a capitalist hub.

But it has been locked in a lengthy battle with Beijing's leaders to push through reforms that could culminate in a direct election of its leader in 2017.

Tension has grown over China's meddling in Hong Kong affairs as well as a proposal that all candidates in the 2017 poll be vetted by a panel stacked with Beijing loyalists, which would essentially keep opposition candidates out of the running.

Anson Chan, a respected former senior Hong Kong official, and Martin Lee, one of the founders of the main opposition Democratic Party, met Biden at the White House last Friday, in one of the most high-profile attempts to flag such concerns internationally.

But the United States must "refrain from interfering", Xinhua quoted an official of the Foreign Commissioner's office in Hong Kong as saying.

The official urged the United States to "proceed discreetly" on Hong Kong affairs to avoid damage to China-U.S. ties as the city is now in a "sensitive period" of political reform.

Earlier in their two-week trip to North America, Chan and Lee also met Nancy Pelosi, the leader of the U.S. Democratic Party in the House of Representatives, and testified before a Congressional panel on China.

"The future of freedom and democracy in Hong Kong is under serious threat," U.S. Senator Sherrod Brown, head of the U.S. Congressional Commission that advocates improved human rights and the rule of law in China, 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," he added, during the session.

China has agreed to let Hong Kong elect its next leader in 2017 in what promises to be the most extensive exercise of democracy on Chinese soil. Specific arrangements, however, have yet to be hammered out, including whether public nominations of candidates will be allowed.

A group of pro-democracy activists has threatened to shut down the city's central business district in a campaign of civil disobedience called "Occupy Central", should Beijing bar a fully democratic poll in line with international norms.

In Taiwan, students prompted by similar fears of increased Chinese interference through a contentious trade pact have occupied the legislature for more than two weeks in protest, and a mass protest has pulled in around half a million people.

(Reporting by James Pomfret; Editing by Clarence Fernandez)