A protester is detained by riot police while attempting to leave the campus of Hong Kong Polytechnic University (PolyU) during clashes with police in Hong Kong, China November 18, 2019. Hong Kong police laid siege to a university on Monday (November 18), arresting anti-government protesters while firing rubber bullets and tear gas to pin back them back. REUTERS/Thomas Peter

Police detain protesters who attempt to leave the campus of Hong Kong Polytechnic University (PolyU) during clashes with police in Hong Kong, China November 18, 2019. Dozens, choking on the tear gas, tried to leave the Polytechnic University by breaking through police lines after a night of mayhem in the Chinese-ruled city. REUTERS/Tyrone Siu

Riot police detain a protester and a medic during pre-dawn police operations at Hong Kong Polytechnic University (PolyU), Hong Kong, China November 18, 2019. Many protesters, dressed in regular clothes and without gas masks, made runs for it, dodging tear gas canisters and sponge grenades, only to be forced back inside. PolyU Campus Radio/via REUTERS

Police detain protesters who attempt to leave the campus of Hong Kong Polytechnic University (PolyU) during clashes with police in Hong Kong, China November 18, 2019. Hong Kong has seen a dramatic escalation of the unrest that plunged the city into chaos for almost six months. REUTERS/Tyrone Siu

Protesters are escorted by police out of the campus of Hong Kong Polytechnic University (PolyU) during clashes with police in Hong Kong, China November 18, 2019. REUTERS/Thomas Peter

Protesters are escorted by police out of the campus of Hong Kong Polytechnic University (PolyU) during clashes with police in Hong Kong, November 18, 2019. REUTERS/Thomas Peter