HONG KONG : Hundreds of thousands of protesters, some waving colonial-era flags and chanting anti-Beijing slogans, staged a pro-democracy rally in rain-soaked Hong Kong Tuesday that organisers say could be the largest since the city was handed back to China.

The march reflects surging discontent over Beijing’s insistence that it vet candidates before a vote in 2017 for the semi-autonomous city’s next leader.

It comes after nearly 800,000 people voted in an informal referendum to demand a free electoral mechanism that allows voters to nominate candidates. The poll irked Beijing, which branded it “illegal and invalid”.

Swarms of people poured out of the city’s Victoria Park, the starting point of the march that will culminate in the skyscraper-packed Central business district.

By 6pm (1000 GMT), at least 200,000 protesters had joined the march, Johnson Yeung, a rally organiser told AFP.

The streets turned into a sea of umbrellas and banners bearing slogans such as “We want real democracy” and “Civil nominations for all”, with one activist urging police to remove barricades lining the protest route for wider space to accommodate the surging crowds.

Flanked by security officials in lime-coloured vests, some protesters also sang the Cantonese version of “Do You Hear the People Sing?” from the musical “Les Miserables”.

“There is a strong desire for genuine democracy that offers choice and competition without (political) vetting,” Anson Chan, a former number two official in Hong Kong who is now a pro-democracy activist, told reporters.

The chairman of the Hong Kong post office union, who marched with the protesters in sweltering and muggy weather, said the city’s government was guilty of kowtowing to Beijing’s diktat.

“This march is not for us, it’s for our children. Without universal suffrage there’s no way to monitor the government,” said Ip Kam-fu.

Organisers expect more than half a million people to join the rally, which would be a record high.

It was impossible to independently verify early numbers but the rally seemed visibly larger than the June 4 rally to mark the 25th anniversary of the Tiananmen Square crackdown, which police said attracted 99,500 people.

Paul Yip, a statistician at Hong Kong University, told AFP he was leading a team of 15 to independently assess Tuesday’s crowd size, a topic of great politically sensitivity.

July 1, a traditional day of protest in the former British colony, marks the anniversary of its handover to China in 1997 under a “One country, two systems” agreement.

That allows residents liberties not seen on the mainland, including free speech and the right to protest.

But there are heightened fears that those freedoms are being eroded.

Among other fears, there has been a series of attacks on media workers in recent months - including the stabbing of a liberal former newspaper editor - while pro-democracy media have complained of massive cyber-attacks.

Concerns increased in June when Beijing published a controversial “white paper” on Hong Kong’s future that was widely seen as a warning to the city not to overstep its boundaries.

“Public sentiment has dropped to the lowest point since 2003. I believe more people will come out,” organiser Yeung said.

The 2003 march saw 500,000 people protest against a proposed national security bill, forcing the government to shelve it.

Two student groups have said they will hold an overnight rally after the march to “occupy” a Central street and an area outside the government headquarters.

One of the group’s leaders, Joshua Wong, said the student rally would be held to vent “anger” towards the authorities but would be peaceful.

Pro-democracy group Occupy Central, which organised the referendum, has said that it will stage a mass sit-in in the city’s business district later this year unless authorities come up with acceptable electoral reforms.

A small group of protesters burned a copy of the white paper and a picture of the city’s leader Leung Chun-ying after they were stopped by police near a flag-raising ceremony to mark the handover on Tuesday morning.

“We should avoid doing anything that may undermine Hong Kong’s stability and prosperity,” Leung said in a speech after the ceremony.

The 10-day unofficial referendum, which ended Sunday, gave three options for the election of the city’s next leader, all of which included the public having some influence on the selection of candidates.

Beijing condemned the vote Monday and accused its organisers of breaching the rule of law.

China has promised to let all Hong Kong residents vote for their next leader in 2017 - currently a 1,200-strong pro-Beijing committee chooses the city’s chief executive.

But it says candidates must be approved by a nomination committee, which democracy advocates fear will mean only pro-Beijing figures are allowed to stand.

A study released on Monday by the Chinese University’s Hong Kong Institute of Asia-Pacific Studies suggested that mistrust of Beijing is growing.

Nearly 44 percent of around 800 Hong Kong residents interviewed for the monthly survey said they did not trust the central government, up five percentage points from May.