HONG KONG — Five hundred protesters were believed to be trapped inside a university in Hong Kong on Monday as police laid siege to the campus.

Earlier, police had forced back protesters armed with Molotov cocktails and other homemade weapons at the Polytechnic University as some tried to escape the campus. The university's deputy director of communications told NBC News late Monday that they believe about 500 people are still trapped inside.

Police surrounded the area on Sunday, using water cannons, tear gas and heavy police vehicles to hold the protesters back in a dramatic escalation of the demonstrations that have swept the semi-autonomous Chinese territory since June.

Protesters try to leave the campus of Hong Kong Polytechnic University during clashes with police on Monday. Thomas Peter / Reuters

Earlier, dozens of protesters tried to break through police lines, Reuters reported. Many, dressed in regular clothes and without gas masks, made a run for it, dodging tear gas canisters and sponge grenades, only to be forced back inside. Some were arrested and tackled to the ground.

Police, who have urged those inside to hand over their weapons and follow their orders, tweeted Monday evening that “a large number of rioters, some of whom are underage, still remain on campus.”

Late Monday, hundreds of supporters of protesters were seen marching towards the university. "Save the students at Polytechnic University" and "Hong Kongers, walk together," they chanted.

“Everybody wants to get closer to the campus and help them tonight," a 23-year-old university student who only gave her first name, Emma, told NBC News. "I don’t believe in the police that students will be fairly treated if they surrender."

"I’m scared that this may become another version of Tiananmen Massacre," she said, referring to the 1989 student-led pro-democracy protests in Beijing's Tienanmen Square that were quashed by the Communist government.

A woman who said her child was trapped inside the university appeared to be wracked with worry.

"I can't sleep at all. ... My son is inside there," said Mrs. Lau, a 51-year-old businesswoman who declined to provide her full name over fears for her safety. "I stopped him before as he was trying to support the students at Chinese University. But yesterday, he told me, 'Mom, I have to go out. If I don't go, am I still a human being?' I know he's old enough to be responsible for what he does."

"The first thing, if I can see him, I will hug him and bring him to have a big feast," Lau added as her eyes filled with tears.

Demonstrators have been protesting for months against what they see as Chinese meddling in Hong Kong's affairs and demanding greater democracy for the semi-autonomous territory.

Local Broadcaster RTHK quoted the student union's acting president, Ken Woo, early Monday as saying that he believed some 70 to 100 had tried to leave the campus, but were forced to go back because of tear gas being shot at them.

NBC News saw about 20 people wearing face masks and plain clothes running away from the university Monday afternoon, using a highway and railway tracks to escape before police arrived.

Police said they were aware of injuries inside the campus and have arranged for ambulances to take those needing medical help to hospitals. They said they have also allowed volunteers with the International Red Cross to enter the campus.

Police said in a press conference Monday they have arrested 154 people, ages between 13 and 54, on charges of unlawful assembly and taking part in a riot, among others, over the weekend.

Chief Superintendent Kwok Ka-chuen called the Polytechnic University campus “a weapon factory."

He said “offensive weapons” such as arrows, Molotov cocktails, metal balls and bricks were launched from makeshift catapults mounted on the roof of the campus. Kwok said a large gas canister, which looked like a homemade bomb, was also seen inside the campus.

On Sunday, a police media officer was also shot with an arrow that penetrated his leg.

The unrest in the international financial hub and former British colony has been causing alarm around the world.

Reuters quoted a senior official in President Donald Trump's administration as saying that the United States condemned the "unjustified use of force" in Hong Kong.

Sen. Ted Cruz, R-Texas, tweeted his support for the protesters Saturday, calling the police response "shameful." Sen. Josh Hawley, R-Mo., tweeted Sunday saying "Beijing is pushing Hong Kong into a state of siege." The Republican senators visited Hong Kong in October.

Protesters prepare to shoot bows and arrows at Hong Kong Polytechnic University on Sunday. Kin Cheung / AP

Local authorities said all schools will be closed Tuesday while kindergartens and special schools will be closed for the rest of the week.

Amid the escalation of violence, protesters achieved a legal victory as Hong Kong's High Court ruled Monday that a British colonial-era emergency law revived by the government to ban protesters wearing face masks was unconstitutional.

Jasmine Leung reported from Hong Kong, and Yuliya Talmazan reported from London. Dawn Liu contributed from Beijing.