Leaders of a pro-democracy student group in Hong Kong have postponed talks with the government after demonstrators got into scuffles with opponents.
The group said the authorities were failing to protect protesters.
Police denied colluding with those who used violence against the protesters, and said they had arrested members of the triad criminal gangs.
Activists are protesting at plans by China to vet election candidates and have been occupying parts of the city.
Hong Kong's leader on Thursday offered talks to defuse the situation after the protesters called for his resignation.
But scuffles broke out as people apparently angry with the disruption caused by the protests tried to dismantle tents and barricades.
The Hong Kong Federation of Students, which was invited to negotiations with the government, said in a statement it had "shelved" the talks. A time and place for the negotiations had never been confirmed.
"The government allowed the mafia to attack peaceful Occupy participants. It has cut off the path to a dialogue, and should be responsible for the consequences," it said.
"The government has not kept its promise. We have no choice but to shelve the talks."
It was not clear whether the statement reflected the position of other groups involved in the protest.
Occupy Central leader Benny Tai told the BBC they were still only considering a boycott of the talks. However, he said police were not protecting the demonstrators against attacks by their opponents and this situation could not continue.
"At this point it's very, very difficult to maintain any sense of dialogue if the government does not stop these things happening to peaceful protesters," he said.
There was no immediate response from the government to the postponement.
But in a late-night news conference, reported by the South China Morning Post newspaper, police gave their account of events.
They denied accusations by the protesters that they had acted in concert with forces trying to break up the protest.
They said 19 people had been arrested, eight of whom had "triad backgrounds", referring to local criminal gangs.
'Just a game'
In the commercial district of Mong Kok, on the Kowloon peninsula, opponents of the demonstrators had tried to dismantle tents.
Police linked arms to try to separate the opposing groups.
Later, more pro-democracy activists flooded the area and by evening vastly outnumbered their opponents, says the BBC's Martin Patience in Mong Kok.
They began chanting: "Go back to the mainland." Many activists suspect that these people are coordinated by the Hong Kong or Chinese governments.
But their very presence there is a reminder that not everyone in Hong Kong is on the side of the protesters, our correspondent says.
At least some of the protesters' opponents appear to be local residents angered by the disruption.
"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, quoted by AFP news agency.
"Give us Mong Kok back, we Hong Kongers need to eat!" another said.
Similar scuffles took place in Causeway Bay on Hong Kong Island, where residents tried to remove barricades put up by pro-democracy protesters.
The UK-based human rights group Amnesty International said that during the clashes police "stood by".
"The authorities have failed in their duty to protect peaceful protesters who came under attack," the group's Hong Kong director Mabel Au said.
She added that there were witness accounts of women being physically attacked in the two areas.
'Doomed to fail'
Government offices in the main protest-hit area have been closed, with the authorities urging staff to work from home because roads were blocked.
At the heart of the row is how Hong Kong elects its next leader. In August, Beijing imposed tight rules on nominations for candidates wanting to stand for election.
The protesters say this move means that the polls will fall short of the free elections they are seeking.
Several days into their week-long demonstrations, the protesters added the demand that Chief Executive CY Leung step down.
Mr Leung refused late on Thursday but offered talks with his deputy, which the activists accepted.
The central government in Beijing has thrown its full support behind Mr Leung, calling the protests illegal and "doomed to fail".
Hong Kong democracy timeline
- 1997: UK gives Hong Kong back to China under a 1984 agreement giving it "a high degree of autonomy" for 50 years
- 2004: China says it must approve any changes to Hong Kong's election laws
- June-July 2014: Pro-democracy activists hold an unofficial referendum on political reform; both sides hold large rallies
- 31 August 2014: China says it will allow direct elections in 2017 but will pre-approve candidates
- 22 September 2014: Student groups launch a week-long boycott of classes
- 28 September 2014: Occupy Central and student protests join forces and take over central Hong Kong
- 2017: Direct elections for chief executive due to take place
- 2047: Expiry of current agreements
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