Hong Kong demonstrators rejected demands to immediately end rallies that have paralysed the city's downtown, braving rainstorms to turn out in swelling numbers before a national holiday Wednesday expected to put their campaign for free elections into overdrive.

Thousands ignored the call from the city's embattled leader to end the escalating sit-in and brushed off Beijing's branding of their demonstrations as "illegal" to hit the streets of the key financial hub, despite thunder and heavy downpours.

Protest leaders are confident of mustering massive crowds, angered at Beijing's refusal to grant full democracy, for the National Day public holiday marking the 65th anniversary of the founding of Communist China.

Persistent downpours sent umbrellas skyward and crowds scurrying for shelter, but the worsening weather left determined participants in the so-called "Umbrella Revolution" undeterred.

"We have spent more than a week under the sun, under pepper spray, we of course can stand the rain. Nothing can stop us," a recent university student who identified himself as Choi told AFP.

Protesters wearing the yellow ribbons that have come to symbolise the movement occupied a growing stretch of a main thoroughfare through the city's financial district overnight, while a new protest site several-hundred strong was emerging at Tsim Sha Tsui, a shopping district on the other side of Hong Kong's world-famous harbour.

- 'Not a party' -

A carnival-like atmosphere continued to hold sway in the streets, with outspoken Hong Kong singer Anthony Wong performing for crowds to show his support.

However, there were increasing calls for demonstrators to show greater seriousness, with organisers heard over loudspeakers urging participants not to drink alcohol.

One sign sternly reminded the student-dominated crowds that the rallies were "a protest, not a party", while another drew sombre parallels with the 1989 demonstrations in Beijing's Tiananmen Square, where weeks of peaceful, festival-like protests were crushed by the Chinese military.

Demonstrators expressed differing views on the Hong Kong movement's success, with some saying the prospect of public holidays would encourage many to stay later than they had in previous nights.

"The government has lost, they just don't know it yet," student Kelvin Yip told AFP.

Others were less optimistic on the protesters' ability to sustain momentum.

"We will have more people come out for the holidays, people that were too busy will join," said protester Emma So.

"I don't know what will happen after, especially for older people, it's hard to protest night after night."

- Challenge to Beijing -

Beijing has been left grappling with one of the biggest challenges to its rule over the semi-autonomous city at a time when the Communist Party is cracking down hard on dissent on the mainland.

In his first public comments since demonstrators were tear-gassed by riot police on Sunday evening, Chief Executive Leung Chun-ying said the pro-democracy sit-in organised partly by the Occupy Central group was now "out of control".

"Occupy Central founders had said repeatedly that if the movement is getting out of control, they would call for it to stop. I'm now asking them to fulfil the promise they made to society, and stop this campaign immediately," he said.

But protest leaders rejected Leung's demands and renewed calls for the Beijing-backed leader to step down.

"I think there will be a massive turnout, over 100,000 people tonight and leading into National Day," hedge fund manager and Occupy Central activist Ed Chin told AFP.

The demonstrations, the most intense civil unrest Hong Kong has experienced since its 1997 handover from British rule, were sparked by Beijing's decision in August to restrict who can stand for the city's top post.

British Deputy Prime Minister Nick Clegg said he would summon China's ambassador to London over the protests to express his "dismay and alarm" about the refusal to grant free elections.

Hong Kongers will be able to vote for their next chief executive in 2017 but only two or three candidates vetted by a pro-Beijing committee will be allowed to stand -- something demonstrators have labelled a "fake democracy" that shows Hong Kong cannot trust its mainland overseers.

- 'Keep resisting for democracy' -

Protesters have two demands -- that Leung step down and Beijing rescind its insistence that his successor be vetted before standing for election.

"If the government does not respond after October 2, the action will inevitably be stepped up," Alex Chow, chairman of the Hong Kong Federation of Students, told reporters.

Influential student leader Joshua Wong, who was detained by police last week, said Hong Kong's leader was the "only one" who could end the protest.

"Everyone here has the power. We're here for the bargaining chip with Beijing," he said.

"Leung Chun-ying and the government are feeling increased pressure."

But analysts say the chances of Beijing backing down are virtually non-existent, leaving a city once renowned for its stability plunged into an unknown future -- with democracy activists concerned the police could return in force at any moment.

Beijing stayed defiant Tuesday, saying it supported Hong Kong's handling of the protests, which it described as "illegal activity".

Hong Kong authorities are caught between protester demands, Beijing's uncompromising stance and efforts to keep the city running.

Business leaders complain that the protests are hitting the economy. The world's top cosmetics group L'Oreal said it was suspending all business travel to Hong Kong due to the demonstrations.