Hong Kong police have arrested eight suspected members of triad criminal gangs over clashes at ongoing pro-democracy protests, after attacks by groups including Beijing supporters left demonstrators injured and bloodied.

The announcement early Saturday by police that they had arrested 19 men, including several thought to have ties to the city's notorious organised crime gangs, came after student protesters called off talks with the government, citing police failure to stop the attacks.

Hong Kong's main student union said it was walking away from negotiations after police appeared to ignore what it claimed was orchestrated violence carried out by paid thugs sent by authorities to stir up trouble, with the aim of discrediting the protesters.

"There is no other option but to call off talks," said the Hong Kong Federation of Students (HKFS), one of several groups driving a campaign for free elections that has brought tens of thousands of people onto the streets of the semi-autonomous Chinese city for a week.

"The government and police turned a blind eye to violent acts by the triads targeting peaceful Occupy protesters," the union added, referring to Occupy Central, another prominent group.

At the press briefing Saturday, police denied acting in concert with triads, adding that 12 people had been injured in the clashes, including six officers.

Hong Kong's embattled leader Leung Chun-ying, facing calls to resign but firmly backed by Beijing, had promised students talks with a top civil servant in an attempt to end the stand-off that has posed the most serious challenge to China's ruling Communist Party in years.

Amnesty International blasted police, saying officers had "stood by and did nothing" to protect protesters. It had first-hand witness accounts of women being attacked in the densely packed shopping districts of Causeway Bay and Mong Kok, the rights group said.

Demonstrators compared the police "inaction" to Sunday, when officers fired pepper spray and tear gas at the peaceful crowds.

"The police are so unfair, these people attack us and they do nothing," Jenny Cheung, a demonstrator, told AFP.

"We protest peacefully and the police use tear smoke and pepper spray; when we are attacked the police do nothing."

Police defend response

Police have defended their response to the chaotic scenes, with senior superintendent Kong Man-keung telling reporters the force had "deployed a lot of manpower to control the situation".

But furious protesters said pro-Beijing thugs had been freely allowed to attack their camps. Crowds in Mong Kok chanted "Bring out the handcuffs!" late into the night.

Police officers were seen escorting a man from the scene with his face covered in blood.

There were widespread allegations of sexual assault in the packed crowds, with three girls seen being bundled into a police van in tears after apparently being assaulted at the Causeway Bay protest.

"I urgently want to express to all citizens, no matter what attitude you have towards Occupy (Central), you still have to remain calm, and not use violence or disrupt order under any situation," the city's leader Leung, viewed by demonstrators as a Beijing stooge, said in a televised message.

The protests were triggered by China's announcement in August that while Hong Kongers can vote for their next leader in 2017, only candidates vetted by Beijing will be able to stand -- a decision dismissed as "fake democracy" by campaigners.

While the United States, Europe and Japan have all expressed their concern at the scenes playing out in the key Asian financial hub, China's Communist authorities refuse to make concessions.

In a doom-laden editorial Saturday, China's People's Daily newspaper warned that the protests would cause "severely disrupted social orders, huge economic losses and possible casualties".

The Communist Party mouthpiece defended the decision by police to fire tear gas on unarmed protesters earlier in the week, describing their actions as "necessary, appropriate and moderate".

'Beat them to death'

Demonstrators had set a deadline of midnight Thursday for chief executive Leung to resign and for Beijing to abandon the proposals to vet candidates.

Leung refused to quit, but in a dramatic televised appearance shortly before the deadline he appointed his deputy to sit down with the HKFS, which has been at the vanguard of the protests.

Mistrust was rife that Leung was merely trying to buy time in the hope that the campaign will lose momentum, with Hong Kong residents tiring of the disruption caused by the daily mass sit-ins.

Friday's clashes broke out as the city returned to work after a two-day public holiday.

"I don't support Occupy Central. We have to work and make money. Occupy is just a game," said a construction worker who gave his name as Mr Lee.

Individuals from both sides pushed and shoved each other as water bottles were thrown, and one anti-Occupy protester chanted: "Beat them to death, good job police!"

Shop owners have told of a massive downturn in business.

Hong Kong Finance Secretary John Tsang warned that if the unrest persists, the city's status as one of the world's most important trading hubs could be under threat.