Around 100 anti-government protesters remain holed up at a Hong Kong university as a police siege of the campus enters its third day. 

Police have surrounded the university and are arresting anyone who leaves. On Monday, groups of protesters made several attempts to escape – including sliding down hoses onto waiting motorcycles. It is not clear whether they managed to evade arrest. 

Later on Tuesday, about a dozen protesters tried to flee through the university’s sewerage system. A Reuters witness who saw them lower themselves into a tunnel wearing gas masks and plastic sheets to cover their bodies.

They were not able to escape and had to retreat back onto the campus.

Hong Kong’s leader Carrie Lam said 600 people had left the Polytechnic University campus, 200 of whom were under 18. 

Those under 18 will not be immediately arrested but could face charges later, Lam said. The 400 other people who have left have been arrested. 

Read More: Everything You Need To Know About The Hong Kong Protests

“We will use whatever means to continue to persuade and arrange for these remaining protesters to leave the campus as soon as possible so that this whole operation could end in a peaceful manner,” she said. 

After five months, the Hong Kong protest movement has steadily intensified as local and Beijing authorities harden their positions and refuse to make concessions.

Universities became the latest battleground last week, as protesters occupied several campuses, using petrol bombs and bows and arrows to fend off riot police backed by armoured cars and water cannon. Those at Polytechnic are the last holdouts.

China, which took control of the former British colony in 1997 promising to let it retain considerable autonomy, flexed its muscles, sending troops outside their barracks over the weekend to help clean up debris strewn by protesters to block streets.

China’s ambassador to Britain accused the UK and the US of meddling in the country’s internal affairs and warned that the Chinese government “will not sit on our hands” if the situation in Hong Kong “becomes uncontrollable”.

When Lam was asked whether she would ask for help from Chinese troops based in Hong Kong, she said her government remains confident it is able to cope with the situation.

China also hinted it might overrule the Hong Kong’s high court ruling on Monday to strike down a ban on face masks that was aimed at preventing protesters from hiding their identity to evade arrest.

Monday’s ruling said the ban infringes on fundamental rights more than is reasonably necessary. The ban has been widely disregarded.