A protester looks at fire at Hong Kong Polytechnic University (PolyU) during an anti-government protest in Hong Kong, China November 18, 2019. Hong Kong police on Monday, November 18, 2019 trapped hundreds of protesters inside a major university, sealing off roads in the area after almost two straight days of standoffs that have raised fears of a bloody showdown with both sides refusing to back down. REUTERS/Athit Perawongmetha

Protesters on a rooftop use a slingshot to throw stones at riot police during clashes outside Hong Kong Polytechnic University (PolyU) in Hong Kong, China November 17, 2019. Hundreds of defiant protesters inside Hong Kong's Polytechnic University started fires at the school's entrance to prevent police from entering, while police water cannon and armoured vehicles were deployed in raging battles that lasted an entire day and into the night. REUTERS/Thomas Peter

Police fires a water cannon during clashes with anti-government protesters, outside Hong Kong Polytechnic University (PolyU) in Hong Kong, China, November 17, 2019. A police officer was shot in the calf by an arrow as anti-government protesters, many of them students, responded to police with salvos of petrol bombs and bricks hurled by homemade catapults. Police threatened to fire live bullets if "rioters" did not stop using lethal weapons in the latest flare-up in anti-government protests that have convulsed the Chinese-ruled city for five months. REUTERS/Athit Perawongmetha

Anti-government protesters flee during clashes with police at Hong Kong Polytechnic University in Hong Kong, China November 18, 2019. Scores of protesters were injured, some with scalding burns from chemicals in the jets fired from the water cannon. The protesters at Polytechnic University had blocked one of Hong Kong's major highways, the Cross Harbour Tunnel linking Hong Kong island to the Kowloon peninsula for much of the past week, with authorities desperate to restore the link yet encountering tenacious resistance from the trapped activists. REUTERS/Laurel Chor

Protesters clash with police outside Hong Kong Polytechnic University (PolyU) in Hong Kong, China November 17, 2019. Thousands of residents and protesters flocked to various districts around the university including Tsim Sha Tsui, Jordan and Yau Ma Tei, to try to penetrate the riot-police lines to rescue the trapped students. REUTERS/Adnan Abidi

A fire is seen at Hong Kong Polytechnic University (PolyU) during an anti-government protest in Hong Kong, China November 18, 2019. In Monday's statement, police warned people whom they described as rioters to stop using lethal weapons to attack officers and to halt other acts of violence, saying officers would respond with force and possibly live bullets if necessary. REUTERS/Adnan Abidi