Pro-democracy protesters were besieged by violent mobs looking to break up their occupation of the busy district on Friday, leading to chaotic skirmishes on city streets, accusations of police bias, and the shelving of negotiations with the government.

Nineteen people were arrested, at least eight of whom had "triad backgrounds", police said, lending weight to claims from pro-democracy groups that the violence, which injured 18, was instigated by gangs who had been tasked with breaking up the demonstrations.

Witnesses said the attackers punched and kicked protesters, while female protesters were groped and harassed.

Regardless of who the instigators were, they attracted a large group of dozens, if not hundreds, of others who joined in surrounding the pro-democracy tent, baying for blood, sometimes literally. "Kill!" was heard yelled from the crowd, while observers who tried to intervene were also threatened physically.

Student organisations leading the largely peaceful democracy protests warned that anti-Occupy, pro-government groups were preparing to ramp up counter-protests, and were seen gathering near the main pro-democracy rally point at government headquarters.

Some of the pro-government groups were wearing blue ribbons, as opposed to the yellow-ribbon campaign that has become synonymous with Hong Kong's pro-democracy campaign.

Protester Dorothy Lui, a 22-year-old public health student at the Chinese University of Hong Kong, said she was worried at the shift in mood among exhausted students, most of whom have been occupying city streets for a week.

"People here are getting less and less rational and impatient because of the past week," she said. The trust between police and students, she said, had been completely broken after last Sunday's deployment of tear gas and anger at perceived inaction from police during the violence on Friday. "It's quite dangerous if a lot of large crowds have mad emotions."

Protesters largely rebuilt the Mong Kok site overnight but the situation remained tense, with about 1000 protesters from both sides facing off on Saturday morning.

Friday's violent scuffles, the ugliest scenes since police fired tear gas on crowds, laid bare divisions within the community over the protests, which have paralysed major city blocks for a week. But it also revealed the depths of support for the exhausted democracy campaigners.

After word spread on social media that the protest site in Mong Kok had been attacked, thousands of people descended on the scene, surrounding instigators of the attack, yelling at police to arrest them.

Standing in the human-chain protecting the pro-democracy protesters, Paul Ng, dressed in a dark-grey shirt and business pants, said he felt compelled to help when he saw what was unfolding.

"My office is right there," the 44-year-old auditor said, pointing to a nearby building. "So I decide to stand here and protect the students." He said he had attended rallies every night and had slept overnight once to show solidarity.

But Liu Liping, who moved from Hunan province in mainland China to Hong Kong seven years ago, said many people in Hong Kong opposed the protests.

"I just live here and we work very hard just to make a living," she said. "They [democracy protesters] don't represent all of Hong Kong."

Hundreds of thousands of people have turned out for pro-democracy protests in the past week after the Chinese government said it would reject calls for Hong Kong to directly elect its next leader. Protesters have been calling on current Hong Kong chief executive Leung Chun-ying to step down.