HONG KONG (AFP) - More than half a million protesters took part in a pro-democracy march in rain-soaked Hong Kong Tuesday, organisers said, a record turnout believed to be the largest since the city was handed back to China.

"This year people came out braving the rain and wind and many citizens joined along the way," Johnson Yeung, a rally organiser, told a cheering crowd in the city's Central business district late Tuesday.

"According to conservative estimates, 510,000 people attended the march."

The scale of the turnout 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 took part in an informal referendum demanding that voters be allowed to nominate their own candidates. The poll irked Beijing, which branded it "illegal and invalid".

The protest route from the city's Victoria Park to the skyscraper-packed Central business district was a sea of umbrellas and banners emblazoned with slogans such as "We want real democracy" and "We stand united against China".

Some protesters sang the Cantonese version of "Do You Hear the People Sing?" - the rabble-rousing anthem from the musical "Les Miserables".

Despite torrential downpours, swarms of protesters continued pouring into the clogged streets through the afternoon and evening.

Police estimated 92,000 took part as of 7.30pm but did not count protesters joining the march midway.

"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, marching in the muggy heat, said the city's government was kowtowing to Beijing.

"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.

The city's Chief Executive Leung Chun-ying sought to strike a conciliatory note, saying his government would do its utmost to forge an agreement on implementing universal suffrage.

He offered no details on the 2017 election when he spoke at a ceremony earlier Tuesday marking the 17th anniversary of the city's handover from former colonial power Britain.

A handful of marchers pushed against police barricades but the rally was largely peaceful, and a carnival atmosphere prevailed in several stretches of the route.

In one street performance, a model tank hurtled towards a protester, pointing its gun barrel at his neck as he attempted to push back.

"Hong Kong is turning into a place with less and less freedom," Eric Wong, a 24-year-old photographer who took part in the rally, told AFP. "It is transforming into the mainland." July 1, a traditional day of protest, marks the city's 1997 handover to China 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 worries, 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 the bounds.

"They (Beijing) promised Hong Kong people a high degree of autonomy. Now they are trying to take everything back," lawyer and veteran democracy campaigner Martin Lee told AFP.

Two student groups have said they will hold an overnight rally after the march to "occupy" a street in Central 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.

The 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 has condemned the vote 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.