China on Monday condemned an unofficial Hong Kong referendum on democratic reform in which almost 800,000 people voted, calling it "illegal and invalid" on the eve of huge pro-democracy protests in the city.

Beijing said the vote went against Hong Kong's constitution and accused its organisers -- local democracy activists -- of breaching the rule of law.

Organised by the activist group Occupy Central, the informal referendum saw more than 790,000 people vote over 10 days on how Hong Kong's next leader should be chosen.

Beijing said the activists were determined to "pursue self-interests through breaching the rule of law, disturbing Hong Kong's social order and holding back the progress of universal suffrage".

The statement, issued by the Hong Kong and Macao Affairs Office of the State Council and carried by China's official Xinhua news agency, said Beijing fully respects Hong Kong people's wish for universal suffrage, but such a wish "can only be realized through lawful, rational and down-to-earth discussions among various sides".

Beijing has promised to let Hong Kong residents elect their next leader in 2017, but has ruled out giving voters a say in selecting candidates.

This has fuelled fears among democracy advocates in the city -- a British colony until 1997 -- that only those sympathetic to Beijing will be allowed to stand. Hong Kong's current chief executive was selected by a 1,200-strong pro-Beijing committee.

Beijing's statement came on the eve of a mass rally in Hong Kong marking the July 1 anniversary of the transfer of sovereignty, which organisers expect to be the largest since the handover with at least 500,000 people expected to take to the streets.

- High tensions -

Students are also planning a peaceful overnight rally in the Central business district as well as outside government headquarters.

"It's time to escalate our action and put pressure on the Hong Kong government and the central government," said student leader Joshua Wong.

Concerns have been growing that the freedoms Hong Kong was guaranteed under the "One Country, Two Systems" deal at the time of the handover are being eroded.

There have been a spate of attacks on media workers in recent months -- including the brutal stabbing of a liberal former newspaper editor -- while pro-democracy media have complained of massive cyber-attacks.

Earlier this month Beijing published a controversial "white paper" on Hong Kong's future that was widely seen as a warning to the city not to overstep boundaries.

Benny Tai, who organised the referendum, said the unexpectedly high turnout in the vote should force Hong Kong's government to act.

"The Hong Kong government should take seriously the views of nearly 800,000 citizens," he told a radio programme on Monday.

The number of voters represents almost a quarter of the 3.47 million registered for elections in 2012, drawn from a city of 7.2 million people.

The referendum -- which took place online and at physical polling stations -- gave people three options to choose from, all of which gave voters some degree of influence over the candidates vying to become Hong Kong's leader.

All are therefore considered unacceptable by China and the Hong Kong government, who say a nominating committee must pick candidates under the terms of the city's mini-constitution.

The winning proposal, offered by the Alliance for True Democracy, would allow the public or democratically elected lawmakers to nominate candidates.

Hong Kong's government said in a statement late Sunday that it respects "the right of the people to express their views and we also understand that there are different views in society".

Occupy Central have threatened a mass sit-in in the Central business district later this year unless authorities come up with acceptable electoral reforms.

"A lot of Hong Kong people are afraid that Beijing is going to tighten control and also take back some of the autonomy of Hong Kong," Ma Ngok, a professor at the Chinese University of Hong Kong, told AFP.

"They are afraid... autonomy and freedom may be lost if they don't speak up."