Almost 300,000 people took part in an unofficial Hong Kong vote on electoral reform Friday, a poll which China has condemned as fears grow that Beijing will backtrack on its promise of universal suffrage.

As of 8 pm local time (1200 GMT), eight hours after online polling opened, 298,268 people had taken part in the informal "civil referendum" which asks participants to choose between three methods to vote for Hong Kong's next leader in 2017.

Occupy Central, the local pro-democracy movement that organised the vote, which will be held over 10 days, said a high turnout would prove the city's determination to win "true" universal suffrage.

The number of participants is already set to outstrip expectations -- organisers had said before polls opened that they were hoping for 300,000 people to vote in total.

Under the "one country, two systems" agreement reached when Hong Kong was handed over from former colonial power Britain to Communist-ruled China in 1997, Hong Kong has guaranteed civil liberties not enjoyed on the mainland, including free speech and the right to protest.

The semi-autonomous city's leader, or chief executive, is currently appointed by a pro-Beijing committee.

China has promised direct elections for the next chief executive in 2017 -- but has ruled out allowing voters to choose which candidates can stand.

Many pro-democrats fear Beijing will handpick the candidates to ensure election of a sympathetic official.

- 'Determined to have true democracy' -

"If we could get a good turnout, that shows that Hong Kong people are really determined to have true democracy," Benny Tai, one of Occupy Central's founders, said at a launch event for Friday's vote.

Polling will continue until June 29 and residents have so far cast their votes online and via smartphones. There are plans to open polling booths around the city on Sunday.

Chinese authorities said Friday that any referendum in Hong Kong on how to elect its leader would not have constitutional grounds and would be illegal and invalid, state news agency Xinhua reported.

Each of the three options in the unofficial vote allows residents to choose candidates for the top job -- something deemed unacceptable by the city's current leader Leung Chun-ying.

"All three options on the ballot are against the Basic Law," Leung said, referring to Hong Kong's constitution.

Occupy Central is planning to paralyse Hong Kong's financial district with thousands of protesters at the end of the year, as a last resort if officials do not allow voters to choose their own candidates.

A high turnout in the ongoing vote could indicate that "there may be more Hong Kong people willing to come out and join the final action, and that will generate sufficient political pressure on the government", Tai said.

The vote comes as concerns grow that the civil liberties enjoyed by Hong Kong -- guaranteed only until 2047 under the "one country, two systems" agreement -- are being steadily eroded.

A spate of attacks on Hong Kong journalists and media executives in recent months, including the savage knifing of the former editor of the liberal Ming Pao newspaper, have raised concerns over press freedom in the city.

The online voting system for the unofficial referendum also witnessed one of the largest denial-of-service attacks in history three hours before polls opened, Dow Jones Newswires reported, citing a web security firm.

"They literally used every trick in the book in terms of how attacks are launched, and then some we had never seen used before," Matthew Prince, CEO of CloudFlare told Dow Jones, adding that his firm has been able to keep the site running in Hong Kong.

Late Friday the voting site was showing an error message which read: "System is under severe attack, only limited services provided."

The polling system, built by the University of Hong Kong and Hong Kong Polytechnic University, also suffered a series of similar cyber-attacks last week.

On Wednesday the leading pro-democracy newspaper Apple Daily blamed Beijing for a massive cyber-attack that shut down its website for several hours.

Beijing's cabinet last week published a controversial white paper reasserting China's control over Hong Kong, triggering angry protests in the city.

It was China's first ever policy document stipulating how Hong Kong should be governed, in what was widely interpreted as a warning to the city not to overstep the boundaries of its autonomy.