The United States on Monday asked Hong Kong's leaders to show restraint after riot police fired tear gas on mass democracy protests, and said it had told Beijing it backs universal suffrage in the territory.

"The United States urges the Hong Kong authorities to exercise restraint and for protesters to express their views peacefully," White House spokesman Josh Earnest told reporters.

"The United States supports universal suffrage in Hong Kong in accordance with the Basic Law and we support the aspirations of the Hong Kong people.

"We have consistently made our position known to Beijing and will continue to do so," he said.

"We believe an open society, with the highest possible degree of autonomy and governed by the rule of law, is essential for Hong Kong's stability and prosperity."

Earnest also said that the position of chief executive in Hong Kong would be given more credibility if the people of the semi-autonomous Chinese city could freely choose its candidates for the job.

Escalating tensions in Hong Kong also boosted an online petition, calling on President Barack Obama to press China to avoid a "second Tiananmen massacre" in Hong Kong, past the signature threshold required to compel a White House response.

The petition was lodged under the White House's "We the People" initiative designed to facilitate the constitutional right of US citizens to petition their government.

When 100,000 people sign up to a petition within 30 days, the Obama administration has said it will respond.

As of mid-Monday, the Hong Kong petition, registered on September 4, had gathered 190,000 electronic signatures -- as global attention is focused on the former British colony, which is gripped with mass democracy protests.

The petition calls on Obama to make clear to the Chinese government that any attempt to crack down on the protests would be opposed and punished.

"We, the democracy-loving people living in the US and other parts of the world, urge you to press the Chinese Government to honor its promise of democratic elections to the Hong Kong citizenry," the petition states.

"Given Beijing's records (sic), we fear a second Tiananmen Massacre will happen in Hong Kong.

We believe that the US has the responsibility to prevent such bloody tragedies from happening."

Hong Kong protesters prepare for new night of clashes
Hong Kong (AFP) Sept 29, 2014 - The ranks of Hong Kong democracy protesters who have paralysed parts of the city swelled into their tens of thousands Monday, digging in for another night of confrontation with police in their campaign for free elections.

The protesters defied government calls to go home, a day after chaotic scenes saw riot police fire tear gas in running battles with angry crowds in one of the biggest ever challenges to Beijing's rule of the semi-autonomous city.

As night fell, thousands of demonstrators who have blocked off a major highway turned on the torches on their mobile phones, turning the Admiralty business district into a sea of lights.

Some of those swelling the crowds on Monday said they had been horrified to see police using tear gas on the protesters -- many of whom are students -- and came to voice their disgust.

"We don't know how the police could do that," teacher Shum Yuen-ping told AFP.

"We want to have our own democracy, and we came to provide our students with support."

The demonstrators are furious over China's refusal to grant free elections for the city's next leader in 2017, insisting that it will vet the candidates in a decision that critics brand a "fake democracy".

Public anger over rampant inequality is also at its highest in years in a world financial hub once renowned for its stability.

- 'Umbrella revolution' -

The tense scenes of Sunday night gave way to something of a festival atmosphere on Monday as riot police retreated, leaving the huge masses of protesters to spread across at least three major thoroughfares around the city.

Cantonese pop filled the air during the second day of what some are dubbing the "umbrella revolution", as protesters have been using them to protect against tear gas and the scorching sun alike.

One British sympathiser won huge cheers as he set up a barbecue and began handing out hamburgers and sausages to the protesters.

"I saw everybody was just standing around and just eating bread and bananas and I thought, 'These guys have been here for 24 hours now, and everybody needs cooked food," Daniel Shepherd, a finance broker by day, told AFP.

"Firing tear gas at students that are unarmed, I think, seems a bit excessive," added the 32-year-old, who has lived in Hong Kong for 13 years.

But many Hong Kongers expressed frustration at the huge disruption the protests have caused, with the crowds blocking key junctions in the busy Causeway Bay and Mongkok shopping districts as well as the biggest protest site in Admiralty.

With several of the city's major arteries at a standstill there was chaos on the transport network on Monday, leaving commuters struggling to get to work and forcing many schools and businesses to close.

The government said schools in two central districts would be closed for a second day on Tuesday.

- Political headache for China -

Analysts say the protests put the Chinese government in an extremely difficult position.

Communist authorities are worried that dealing with the protests too softly could spark wider protests for greater freedoms on the mainland -- but a heavy-handed response could spark an international outcry.

Hong Kong stocks ended down 1.90 percent on Monday as investors fretted about the impact the ongoing disruption could have on a key regional market.

Beijing moved swiftly to wipe mentions of the protests from Chinese social media -- blocking photo-sharing service Instagram altogether -- and reiterated its hardline stance, opposing the demonstrators' "illegal actions".

But with many protesters vowing to stay put until they win democratic reforms, there is no obvious way out of the deadlock.

"The difficulty is that there seems to be no going back for both sides," said Surya Deva, a law professor at the City University of Hong Kong.

"Which side will blink first is difficult to say, but I think protestors will prevail in the long run."

Hong Kong's leader Leung Chun-ying dismissed rumours circulating on social media that he planned to call in the Chinese military, which stations a garrison in the city.

"There is absolutely no proof of this," he said.

Underlining their assertive stance, some 1,000 masked protesters gathered outside a police station where senior officers held a press conference defending their liberal use of tear gas against crowds on Sunday night.

Assistant Commissioner Cheung Tak-keung said tear gas was used "87 times" at nine different locations.

"Force is used in a situation when we have no other alternatives," he said, adding officers were compelled to deploy the gas when "police cordon lines were heavily charged by some protesters".

But lawyers from the Hong Kong Bar Association condemned "excessive and disproportionate use of force" against crowds which it said were clearly predominantly peaceful.

Police said 41 people have been injured, including 12 officers in the past few days, and 78 arrests made for offences ranging from forcible entry into government premises and unlawful assembly to assaulting public officers.

The mass protests came after a week of increasingly tense student-led demonstrations that saw people mob the city's leader and storm the central government complex.