Thousands of Hong Kong pro-democracy protesters massed to block entry to the offices of their city's embattled leader early Thursday in an attempt to force a dialogue, as calls for his resignation grew louder.

More than 3,000 protesters gathered outside Leung Chun-ying's office in the early hours, chanting for the chief executive -- seen by critics as a Beijing stooge -- to step down, while police equipped with riot helmets and shields stood firm behind barricades.

"We're trying to surround the entire government complex and wait for CY (Leung) to come back to work on Friday (after Thursday's public holiday)," protester Thomas Choi told AFP.

"We want to talk to him face to face."

Four days of peaceful demonstrations have seen tens of thousands of people take over key districts and highways in Hong Kong as they call on Beijing to grant free elections to the semi-autonomous city.

On Wednesday night, one of the student leaders organising the protests threatened to step up the action -- including a possible attempt to occupy government offices -- if Leung did not resign by Thursday.

"We will consider having different operating actions in future days, including occupying other places like important government offices," said Agnes Chow of student movement Scholarism.

Students have been at the forefront of the demonstrations but others have expanded their ranks since riot police tear-gassed protesters on Sunday night, in chaotic scenes that triggered an outpouring of support.

With Wednesday and Thursday both public holidays in Hong Kong, many workers have been free to swell the masses in the streets.

In a sign of Beijing's growing unease, a local tourism leader said Chinese travel agents were reporting that group visits to the city had been suspended.

"I'm hearing this from a lot of travel agents in China," Joseph Tung, executive director of the Travel Industry Council of Hong Kong, told AFP, stressing that the move had not been confirmed by Beijing.

October 1-7 is known as "Golden Week" in mainland China, a key shopping holiday that sees many travel to the southern Chinese city.

- 'We need to escalate' -

The standoff at Leung's offices came after vast crowds poured onto the streets for a fourth sweltering night.

There was a mixed response to the call to ramp up the action.

"We need to escalate the movement," said 23-year-old student Jason Chan. "So many people have come out every day and the government hasn't responded to us. If we don't take things to the next level, this movement is pointless."

But many others were reluctant to take any action that could prompt further clashes with the police.

"I think we should keep this a peaceful revolution," said costume designer Janice Pang.

"Hong Kong people may not support us if we do something more extreme."

The protesters are furious at the central government's refusal to allow free elections for the city's next leader in 2017, insisting that only two or three candidates vetted by a pro-Beijing committee will be permitted to stand.

They call this "fake democracy" and have two demands -- that Leung step down and that Beijing reverse its decision.

In a movement being dubbed the 'umbrella revolution' -- a nod to the umbrellas they have used to protect themselves against tear gas, the sun and torrential downpours alike -- the protesters have brought key parts of the city to a standstill, heavily disrupting the transport network and shutting down businesses.

- China tells US to back off -

China on Wednesday issued a stern warning to the Washington to stay out of the situation, brushing off US calls for restraint and saying the protests were none of its business.

"The Chinese government has very firmly and clearly stated its position. Hong Kong affairs are China's internal affairs," Chinese Foreign Minister Wang Yi told US Secretary of State John Kerry ahead of talks.

"All countries should respect China's sovereignty and this is a basic principle of governing international relations," Wang said.

However, support for the protesters has been growing worldwide, with thousands rallying Wednesday to support the protests.

About 4,000 people turned out in Taipei to rally in solidarity with the Hong Kong demonstrators, while in London 2,000 people gathered outside the Chinese embassy on Portland Place in central London holding up black and yellow umbrellas.

Huge crowds in Hong Kong cheered late Wednesday as messages of support from around the world were projected onto the main government building -- including words of encouragement from sympathisers in mainland China.

The protests pose a huge political challenge for Beijing at a time when the Communist Party is cracking down hard on dissent on the mainland.

Authorities have scrubbed mentions of the protests from Chinese social media, while rights groups say more than a dozen activists have been detained and as many as 60 others questioned for expressing support for the Hong Kong crowds.

The last British governor of Hong Kong said Wednesday that the entire situation had been "very, very badly mishandled", calling on authorities to enter into consultation with demonstrators.

"I think we've got to see dialogue replacing tear gas and pepper spray," Chris Patten told the BBC radio, adding that China was reneging on its promises to allow the city to manage its own affairs.