China's refusal to allow free elections in Hong Kong risks an open-ended confrontation that will test how far Beijing will go to stop the city's pro-democracy fever from infecting the mainland.

While a heavy-handed response threatens the city's reputation as a stable, world-class business hub, Beijing fears that unchecked protests could spill across the border and ignite discontent with one-party Communist rule.

"China is watching this very nervously," said Michael Kugelman, an Asia expert with the DC-based Woodrow Wilson International Center for Scholars, "We are getting close to an inflection point."

The pro-democracy movement in Hong Kong gained new momentum Sunday night as a unexpectedly tough police crackdown galvanised support for the protests and brought more people onto the streets.

Many protesters have expressed fears that China is tightening its grip on the city of seven million and that cherished freedoms not enjoyed on the mainland are under threat.

Despite US-led calls for restraint after wild scenes when police fired volleys of tear gas at the crowds, China said it "fully" backs Hong Kong authorities in their handling of the demonstrations.

"We oppose all illegal acts in Hong Kong," foreign ministry spokeswoman Hua Chunying said Tuesday.

The protestors have vowed to remain on the street until Beijing revokes its decision to restrict who can stand in the city's next leadership election, and introduce full universal suffrage.

Since the weekend the gatherings have been large but peaceful, although the protesters remain wary as few expect Beijing to offer any concessions, raising questions about the next steps for the movement -- and for China's rulers.

The Communist Party is concerned that allowing the protests to continue could see a contagion on the mainland, which has witnessed a clampdown on dissent under the leadership of Xi Jinping.

Since he came to power, the party has arrested scores of activists, journalists, academics, lawyers and others it sees as a threat to its rule, in what rights groups say is the harshest such crackdown in decades.

Beijing authorities on Monday directed all websites to "immediately" remove any information related to the Hong Kong protests, according to the US-based website China Digital Times which monitors Chinese propaganda.

The photo-sharing service Instagram was suspended, searches for terms such as "Occupy Central" and "Hong Kong students" were blocked from the Twitter-like Weibo and government censors swiftly erased content critical of Beijing from Chinese social media.

"China certainly still has the capacity, ability and desire to control the type of information that gets out there," said Kugelman.

'Fragile moment'

The protests nonetheless put the Chinese government in an extremely difficult position.

"What happens there not only concerns the future of Hong Kong, but reflects China's future as well," prominent Chinese activist and artist Ai Weiwei said in an interview with CNN Monday, describing it as a "fragile moment".

Hong Kong leader Leung Chung-ying struck an uncompromising tone Tuesday, demanding an immediate end to the rallies and warning there would be no budging on the election issue.

"In Hong Kong, they (the local government) can't do much because Beijing is trying to control it step by step," said Surya Deva, associate professor at the School of Law, City University of Hong Kong.

Analysts warn any attempt to disperse the protests by force would lead to an escalation that could seriously damage Hong Kong's image as a reliable Asian financial hub.

The freewheeling city built its position as one of the world's centres of capitalism thanks to transparency, ease of doing business and the rule of law.

And while its economy is a fraction of the size of China's -- a dramatic shift from when Hong Kong was returned to China in 1997 -- the semi-autonomous city remains the preferred gateway for many international businesses.

"Hong Kong is still thought of very strategically by the powers in Beijing. It is still a powerful financial centre, so it is unlikely that China would see it as something that they can just relegate to the back-burner," said Kugelman.

Business leaders have repeatedly called for dialogue to end the impasse -- but with hardline Beijing unlikely to make any concessions it is not clear what dialogue could achieve.

"This is a self-initiating movement," said Ivan Choy, lecturer at the Chinese University of Hong Kong. "There is a strong motivation from people to stay on the street."

The international community is closely watching events in Hong Kong, and a heavy-handed clampdown would quickly draw condemnation.

"China projects itself as a responsible international player in a way that they didn't in '89," says Kugelman, referring to China's bloody suppression of the Tiananmen Square protests -- an event which generates huge protests in Hong Kong on the June 4 anniversary.

But that does not rule out the potential for a surprise flare-up with spiralling consequences.

"If violence is used against them (the protestors) then all bets are off," added Kugelman.