Thousands of pro-democracy protesters forced their way past police and poured on to a main road in Hong Kong late on Sunday night, escalating their direct action campaign.

But within hours, officers using dogs and wielding batons and an apparently powerful pepper-based spray had forced demonstrators out of the road tunnel they had barricaded, chasing them into a nearby park in chaotic scenes close to government offices and the main protest zone in Admiralty.

There were injuries on both sides and multiple arrests amid the clashes, with police and protesters seemingly braced for a long and unpredictable confrontation.

Student organisations had urged supporters to step up the protests by surrounding government headquarters. Unable to reach them due to barricades and a heavy police presence, protesters broke through cordons into Tamar Park and Lung Wo Road, closing the four-lane east-west route.

“Yes, we are escalating. It is how civil disobedience works: if one action is not enough to get government attention we must fight harder,” said Kenci Wong, 24, who works in advertising.

“The police have been beating us so many times. But what we are asking for is right. The government hopes we will get tired but we are very determined.”

Demonstrators on the frontline wore helmets, masks and goggles, carrying plastic shields and the umbrellas that gave the pro-democracy movement its name. But some were left bloodied as officers tried to repel them.

The pepper solution sprayed by police in recent days appears to be stronger than standard pepper spray or even teargas, with a photographer saying he was temporarily blinded by it. Injured protesters with red rashes were washed down with bottled water. One of those giving first aid, a 23-year-old nurse who gave his name as Vincent L, said the spray was six times stronger than teargas and that some of the components were found in pesticide.

A protester is washed down after coming into contact with the police force’s pepper-based spray. Photograph: Kin Cheung/AP

Police have said the solution is “relatively mild” but have not said exactly what it contains.

“Pain is temporary. We are fighting for a permanent democracy,” said Cheuk Pin, 22, a TV programming assistant who had been hurt by the spray.

Protesters put up barriers to block the entrances to the road tunnel through which the city’s chief executive, Leung Chun-ying, drives to work each morning.

But soon afterwards they fled as police launched their charge.

Pro-democracy protesters abandon the main road outside the government headquarters in Hong Kong. Photograph: Kin Cheung/AP

Protesters have called for Leung’s resignation as well as open nominations for the election of his successor. The movement was sparked by Beijing’s decision to impose tight controls on nominations in 2017, but authorities say offering universal suffrage is in itself a step forward.

Neither side has shown any sign of compromise. The resurgence of the movement, which has now entered its third month, follows the authorities’ clearance of some protest areas.

The main occupation in Admiralty has been largely untouched, but a strip close to government offices was cleared earlier.

The Hong Kong Federation of Students called for the escalation in response to police use of violence in removing people and barricades from a protest zone at Mong Kok earlier this week. More than 100 protesters were arrested, including high-profile student leaders Joshua Wong and Lester Shum. Police said they used the minimum level of force necessary.

Many protesters have remained in the district and there were small-scale clashes on Sunday, with a police spokesman saying it remained a “high-risk” area.

Some protesters briefly occupied Lung Wo Road in the early days of the movement but many others tried to stop them, concerned that it would alienate public support.

It was occupied again in October, but police used teargas to clear demonstrators and arrested 45 people.

Eighteen-year-old Suki Lau said: “Before, we thought that occupying the tunnel was too radical, but the government has led us here until now we have to escalate our action. I am a little bit scared, though.”

Eric Wong, 17, added: “I got pepper-sprayed last Friday and I know how much it hurts. But I am here to defend my future and the future of Hong Kong. I have been boycotting classes since 28 September because this is all we can do to obtain universal suffrage.”

Kong Man-keung, of the police public relations branch, condemned student organisations for inciting illegal actions and said there were also people using the internet to encourage others to plan violent charges of police lines.

“This sort of incitement would lead more radicals and troublemaking individuals or organisations to take part, making the situation even more chaotic … Untoward incidents can be sparked off any time,” he told a press conference.

He added: “Police urge the illegal road-occupiers not to do any provocative acts, obstruct or charge police officers and maintain a safe distance from our officers. Protesters, particularly students, should stay away from the radicals and troublemakers. Police do not want to see anyone, particularly students, getting hurt.”

Lee Cheuk-yan, a pan-democrat legislator, said: “People are very angry that the government keeps refusing to respond and only use police violence instead of finding a political solution. They should stop challenging the students and other demonstrators by betting on how long they can stand to protest in the street. They must listen to the people.”