WASHINGTON (AFP) - Washington backed calls for voters in Hong Kong to be given a say in nominating their next leader after half a million protesters marched through the city's streets on Tuesday.

"We support Hong Kong's well-established traditions and basic law protections that include internationally recognized freedoms such as freedom of peaceful assembly and freedom of expression," State Department deputy spokeswoman Marie Harf said.

"We believe that an open society with the highest possible degree of autonomy and governed by the rule of law is really essential for Hong Kong's stability and prosperity," she told reporters, risking the ire of China.

Waving colonial-era flags and chanting anti-Beijing slogans, Tuesday's protest was the largest since Hong Kong was handed back to China by Britain in 1997, organizers said.

The scale of the rally reflects surging discontent over Beijing's insistence that it vet candidates before a vote in 2017 for the semi-autonomous Chinese city's next leader.

"I know details about the election process for the chief executive in 2017 are still being worked out," Ms Harf said.

"But we believe that the legitimacy of this person will be enhanced if universal suffrage is fulfilled and if the election provides a genuine choice of candidates that are representative of the voters' will."

Some 800,000 people have taken part in an informal referendum demanding that voters be allowed a say in the nomination of the 2017 candidates, in a poll branded by Beijing "illegal and invalid."