Pro-democracy protests in Hong Kong must be cleared by Monday morning, its chief executive has announced.

Hours after he spoke, however, tens of thousands of people flooded into the Admiralty area of the city centre in the biggest gathering for days. The rally was called to oppose attacks on protesters by opponents of the movement on Friday, and came six days after police used pepper spray and teargas in failed attempts to disperse the crowds.

“Even after all these incidents, it shows that the more they suppress us, the more we will fight,” the student leader, Joshua Wong, told the gathering. The crowd chanted for democracy and roared the unofficial anthem of the movement by the band Beyond.

Many also voiced concern about what would come next. Hong Kong’s chief executive, Leung Chun-ying, who has refused demands for him to step down, said in a televised address it was urgent that all entrances to government headquarters were clear by Monday so that staff could work and roads unblocked so that schools could reopen.

He said if the conflict between pro-democracy and anti-Occupy Central groups continued, it would be “very likely to keep getting out of hand” and urged citizens to keep calm. He added that the Occupy movement had seriously affected people’s lives, incomes and public services.

The former Democratic party legislator, Law Chi-kwong, urged the public not to go to Admiralty and warned that if they did they should be prepared for attempts to disperse them. He told the South China Morning Post the government might act “no later than tomorrow, or even earlier”.

The heads of universities committee issued an appeal to students to leave all Occupy rallying areas and put safety first, but many said the violence had motivated them to attend.

“It’s so outrageous about the teargas first and then the violence in Mong Kok … We believe if we don’t come out we will not be able to voice our opinions as freely,” said Keith Wong, 45.

Yanki Wong, an NGO worker in her 30s, said people needed the rally to raise their spirits so they could carry on the movement, but that she feared numbers would soon dwindle because people would need to return to work after the weekend. Asked what she thought lay ahead, she said: “I don’t want to think about it because I don’t want to be frightened. I just want to go on step by step.”

Earlier, the secretary for security, Lai Tung-kwok, angrily denied accusations that the government had ignored or even condoned violent attacks on pro-democracy protesters on Friday night. Police confirmed, however, that of 19 men arrested during clashes in Causeway Bay and Mong Kok, two of the city’s busiest shopping districts, eight had triad backgrounds. They were detained for illegal assembly.

Protest leaders called off talks with the government after the attacks on Friday night, complaining that police had stood by as men trying to remove protesters hit, punched and sexually assaulted them. More than 50 people were injured, officials said.

Thin police lines tried to hold back men set on taking swipes at other demonstrators. Officers said reinforcements had been unable to reach Mong Kok sooner because of barricades the protesters had erected.

Public broadcaster RTHK reported security secretary Lai as saying: “I am aware that people have said the government has turned a blind eye towards the triads, or even was co-operating with triads. These accusations are invented and very excessive.” He added that the police had “faithfully, truthfully” enforced the law.

The Hong Kong Federation of Students has claimed that the government and police “connived” in the assaults on the protesters – allegations also made by Occupy Central founders and pan-democratic legislators. James To, deputy chairman of the legislative council’s security panel, said the government “used organised, orchestrated forces and even triad gangs in [an] attempt to disperse citizens”, according to the South China Morning Post.

Hundreds more protesters gathered in Mong Kok on Saturday afternoon, saying they had come because they feared further attacks on the site. A smaller group of counter-protesters formed in the same area and there were heated exchanges.

Both the pro-democracy protesters and those who are opposing them have claimed that the other side was being paid to take to the streets.

Police officers, who were present in higher numbers than on the previous day, tied a man’s hands and escorted him away after he was chased and surrounded by protesters claiming he had hit one of them without provocation.

One man in his 50s, who asked that his name not be used because he is a civil servant, said he was there to back the students. “They have to face the government, the police, closed-minded people who are against the protests and those who are paid to do violence just to threaten them and drive them from the streets,” he said. “I was in Causeway Bay yesterday and I saw the hooligans who were relentlessly using force, spilling other people’s blood.

“You can arrest people for illegal assembly. No problem. They are prepared to face the legal consequences. You can’t ask people to use violence against them. You see the students using only their bodies to keep away violence: they have no weapons. They are just raising their hands.”

The mass movement was sparked by Beijing’s plans to maintain tight control over the election of the next chief executive in 2017. The government says the introduction of universal suffrage is a step forward, but protesters complain they have been cheated and are being given only “fake” democracy because Beijing will determine the nominations.

Student demonstrators and supporters of a broader civil disobedience campaign gained wider support after police used teargas and pepper spray in failed attempts to disperse them last weekend. Others in the city have complained about the inconvenience the protesters have caused. Many Mong Kok residentsc riticised the disruption caused to their lives by the occupation of a busy crossroads.

But a demonstrator, Lavine Ho, a 26-year-old hospital worker, said: “People should understand what students are fighting for: for our future and democracy. That affects all of us.”

Cheung Tak-keung, Hong Kong’s assistant police commissioner, said his forces might yet arrest more people over Friday’s violence. The priority had been to separate the opposing groups, the South China Morning Post reported him as saying.

Student leaders have said the government must provide a fuller explanation of what happened in Mong Kok before they will reconsider talks with the chief secretary, Carrie Lam.