Police made a total of 19 arrests over the clashes, with at least eight of those detained believed to have triad backgrounds, a police press conference heard.
After days of peaceful demonstrations, ugly scenes broke out Friday at protest camps, sparking speculation that paid thugs had been sent by authorities to stir up trouble, with the aim of discrediting protesters.
Police denied acting in concert with triads, the press briefing heard.
Violence in several Hong Kong districts left at least 12 people injured including six police, according to authorities, while several women said they had been sexually assaulted.
Protesters call off talks
Hong Kong student leaders have called off talks with the government aimed at bringing an end to mass pro-democracy demonstrations that have paralysed the city, after violent clashes broke out with pro-Beijing crowds at their protest camps.
The Hong Kong Federation of Students made the announcement after pro-government crowds descended on two of their camps on Friday, tearing down tents and barricades in what activists said as orchestrated violence by paid thugs from "triad" criminal gangs.
"There is no other option but to call off talks," the students said in a statement.
"Everybody saw what happened today," they added. "The government and police turned a blind eye to violent acts by the triads targeting peaceful Occupy protesters".
The embattled government of Hong Kong leader Leung Chun-ying had promised talks in a bid to end the protests that have brought swathes of the semi-autonomous Chinese city to a standstill since Sunday.
There were angry scenes in the packed Mong Kok and Causeway Bay shopping districts as pro-democracy protesters faced off with large crowds of opponents, with police struggling to keep the situation under control.
But protesters reacted angrily to the lack of arrests, saying pro-Beijing thugs had been freely allowed to attack their camps.
The 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 faced covered in blood.
There were widespread allegations of sexual assault in the densely-packed crowds, with three girls wearing plastic rain ponchos seen being bundled into a police van in tears after apparently being assaulted at the Causeway Bay protest.
The protesters have massed on the streets in fury at 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.
Leung -- seen by the protesters as a Beijing stooge - refused a demand by protesters to quit but appointed his deputy to sit down with a prominent students' group at the vanguard of the protests.
Friday's clashes broke out as the city returned to work after a two-day public holiday.
“Give us Mong Kok back, we Hong Kongers need to eat!" yelled a man removing barricades.
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!"
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.
He added that extended protests could seriously dent "confidence in the market system in Hong Kong -- that would bring permanent damage that we could not afford".