Hong Kong's security chief furiously denied the government had used triad gangs against pro-democracy protesters Saturday after accusations thugs were hired to stir up violent clashes.

Two of the city's busiest shopping districts descended into chaos on Friday as groups targeted protesters who have brought parts of Hong Kong to a standstill for the past week.

The protesters are demanding the right to nominate who can run as their next leader in 2017 elections, while Beijing has said only candidates it has approved will be able to stand.

Tensions remained high throughout Saturday at the three main protest sites where democracy activists have held sit-ins for the last week as organisers reinforced barricades and set up lookout points in case of further assaults by counter-demonstrators.

Sporadic clashes broke out throughout the day in Mongkok, a densely packed district of shops and apartments that saw some of the worst scenes of violence on Friday night.

There have been widespread reports of sexual assault in the packed crowds, with three girls seen being bundled into a police van late Friday in tears after apparently being assaulted at a second protest site in the Causeway Bay commercial district.

Journalists have also been targeted, with the Hong Kong Journalists Association saying it had confirmed five attacks against members of the media.

China has accused democracy campaigners of destabilising the city and on Saturday an editorial in Communist Party mouthpiece the People's Daily said they were "daydreaming" over the prospect of change.

Of 19 people arrested during Friday's clashes, police said eight were suspected triad members and Amnesty International blasted officers who "stood by and did nothing" to protect protesters.

The city authorities denied any suggestion they had worked with triads.

"I have noticed that people have said the government has turned a blind eye towards the triads or even was cooperating with triads," an angry Secretary for Security Lai Tung-kwok told reporters, raising his voice.

"These accusations are made up and are very excessive," he said.

Police also denied the allegations.

"It is totally unfounded and extremely unfair to police officers who faithfully and diligently performed their duty," Assistant Commissioner Cheung Tak-keung told reporters.

But democrat lawmaker Albert Ho said the police "seemed to show a lot of indulgence to triad activities".

"I have every reason to believe that must be the only option left to those in power in Hong Kong to see that the protesters are cleared from the public area," he told AFP.

"The only way is to allow someone else to do it, short of doing it themselves," Ho said.

Fellow democrat James To also pointed the finger at the authorities.

"I cannot believe that the experienced Mongkok police could not identify triad gangsters," he was quoted as saying by the South China Morning Post.

"The government has used organised, orchestrated forces and even triad gangs in an attempt to disperse citizens."

Triad gangs have traditionally been involved in drug-running, prostitution and extortion but are increasingly involved in legitimate ventures such as property and the finance industry.

Some are believed to also have links with the political establishment and there have previously been allegations of triads sending paid thugs to stir up trouble during protests.

Fears of intimidation have replaced a previous sense of unity after Friday's ugly backlash, heightened by the allegations of sexual assault and the deliberate targeting of journalists.

On Saturday, an AFP reporter in Mongkok heard a female counter-demonstrator tell pro-democracy crowds through a loudspeaker: "Women are supposed to be touched by men." She spoke in Cantonese with a mainland accent.

Meanwhile Hong Kong's Foreign Correspondents' Club said it was "deeply disturbed" by alleged attacks on reporters covering the protests, "especially those ascribed directly to police officers, or where police could have intervened but do not appear to have done so".

The violence prompted student protest leaders to scrap talks with the government, scuppering hopes of a resolution to the crisis.

Demonstrators compared the police "inaction" during the clashes to scenes last Sunday, when officers fired tear gas at peaceful crowds.

Pro-democracy organisers have called on supporters to rally against the attacks at 8:00 pm (1200 GMT) in Admiralty, the largest demonstration site opposite the city's government headquarters.

At an earlier press briefing Saturday, senior superintendent Patrick Kwok said 12 people were injured in the clashes, including six officers.

Officials have blamed both sides for the escalation with Hong Kong's leader again calling on protesters to disperse.

"The government strongly condemns all violent acts," Chief Executive Leung Chun-ying said in a television broadcast.

"There are problems in society, but the right way to solve them is through rational communication, to seek common ground instead of protesting on the streets and allow the problem to get worse."

Friday's clashes broke out as the city returned to work after a two-day public holiday with store owners and delivery drivers frustrated after a week of disruption to business.