Fresh scuffles erupted between pro-democracy activists occupying central parts of Hong Kong and riot police, as the protests entered their second week.
Thousands of people held a rally overnight, defying the Beijing-backed authorities, although by Sunday morning many of the protesters had gone home.
On Saturday Hong Kong’s leader warned that police would ensure government offices and schools reopened on Monday.
Activists oppose China’s plans to vet candidates in 2017 elections.
During the overnight rally thousands chanted: “Democracy now! Democracy in Hong Kong!” as speakers from the pro-democracy movement urged them to persist in their campaign, AP news agency reported.
The scuffles occurred in the early hours of Sunday in the Mong Kok district, with police using pepper spray against some of the protesters.
The comment by Hong Kong leader CY Leung came after earlier street fights led to the postponement of talks between the government and the protesters.
Mr Leung said he “strongly condemned” the violence but warned that it was likely to continue unless “social order” resumed.
“The government and the police have the responsibility and resolution to take all actions necessary to resume social order,” he said.
He added that the “most urgent thing” was for protesters to allow government staff to return to work and to clear main roads so schools can reopen on Monday.
The BBC’s John Sudworth in Hong Kong says that although he did not explicitly threaten to clear the streets by force, CY Leung’s message sounded every bit like an ultimatum.
The Hong Kong Federation of Students withdrew from planned negotiations on Friday, accusing the government of allowing gangs to attack protesters, a claim denied by Hong Kong’s security chief, Lai Tung-kwok.
Police on Saturday said they had arrested 19 people who had been involved in the fighting, adding that eight of them had “triad backgrounds”.
Correspondents say triad gangs have traditionally been known for drug-running, prostitution and extortion networks but have in recent years become involved in legitimate ventures like property development and finance.
Some are also believed to have links with the political establishment, fuelling accusations that they have been paid by the authorities to stir up trouble.
What began as a peaceful sit-in to demand democracy on 22 September escalated last Sunday when riot police used tear gas against unarmed students converging on the government headquarters.
The police response was widely condemned as an overreaction, prompting thousands of people to join the rallies and block key areas, including Hong Kong’s financial district.
At the heart of the row between the protesters and the government is China’s insistence on tight rules on nominations for candidates wanting to stand for election in 2017.
The protesters say the restrictions mean the polls will fall short of the free elections they are seeking and have called for Mr Leung to step down.
But the central government in Beijing has thrown its full support behind Mr Leung, calling the protests illegal and “doomed to fail”.