Hong Kong police and firemen have entered a university which had been under police siege for more than 10 days despite protesters’ pleas for them not to go in.

The operation came less than nine hours after a masked protester told local media that about 20 people were still holed up on campus and urged the police not to enter the site because most were terrified. The protester said they did not trust the police and could not rule out stepping up their action if police stormed the campus.

Early Thursday, police sent in a safety team which comprises explosive experts and firemen to collect evidence and to handle hazardous substances at the Hong Kong Polytechnic University. They also brought in negotiators, government social workers and clinical psychologists in case they found protesters who remained holed up on campus.

Explosives officers look at chemicals while searching for any remaining protesters hiding at the Hong Kong Polytechnic University. Photograph: Anthony Wallace/AFP via Getty Images

At a media briefing, assistant commissioner of police Chow Yat-ming insisted the operation was not to make arrests, but to handle flammable and corrosive materials, including a large number of molotov cocktails, and to gather evidence of what police called a crime scene. He said no riot police and no undercover officers would be deployed.

“On humanitarian grounds, we’re hoping to handle this through peaceful and flexible means, the ultimate goal is the safety of the campus,” he said.

“The major concern is not arrest but the wellbeing of [people inside]. If we encounter anyone, we will try to convince them to accept medical treatment, we’ll record [their identities] and handle it later,” he said.

Media footage shows the campus in a state of chaos, with firemen removing bricks from parapets and picking up glass bottles that were apparently previously filled with flammable liquid. Bricks, rubbish, umbrellas and even a bow were strewn across the college grounds and piled up chairs and other furniture used as barricades were seen everywhere.

The operation came as Donald Trump signed legislation backing the protesters in Hong Kong. “I signed these bills out of respect for President Xi, China, and the people of Hong Kong. They are being enacted in the hope that Leaders and Representatives of China and Hong Kong will be able to amicably settle their differences leading to long term peace and prosperity for all,” Trump said in a statement on Wednesday.

The Hong Kong government said it ‘“strongly opposes and regrets” the legislation and that it would damage the city’s relationship with the US. The government added that the legislation interfered with Hong Kong’s internal affairs and would send the wrong signal to protesters.

At the campus, university senior staff and pro-democracy lawmakers who had inspected and searched the site said on Wednesday they believed no one was left inside and urged the police to end the siege as soon as possible.

Police search the polytechnic campus. Photograph: Nicolas Asfouri/AFP via Getty Images

The protester who spoke to the media demanded that police retreat from around the campus and not register their identities.

Hundreds had been holed up at the Polytechnic University for days after violent clashes broke out between protesters and police on 16 November. The red-brick campus was taken over by demonstrators as they tried to block the neighbouring cross-harbour tunnel as part of a wider plan to paralyse traffic across Hong Kong.

After intensive fighting in which police used teargas and rubber bullets and protesters threw Molotov cocktails and fired arrows at police, officers locked down the campus, announcing that anyone coming out would be immediately arrested for taking part in a riot.

Hundreds of protesters tried to leave, only to be beaten back by teargas, rubber bullets and water cannon. After dozens were caught, they sought more desperate measures, including crawling through sewage and storm water drains, abseiling down ropes, jumping from bridges and escaping on to train tracks.

Police this week softened their tone and said those who came out would not be immediately arrested but would have their identities registered by police, who would investigate them later. More than 1,100 are said by police to have left the campus after having their identities registered. Dozens are believed to have escaped