Hong Kong police said on Tuesday that they are willing to use a peaceful solution and adopt a humanitarian approach to resolve the violence in the Hong Kong Polytechnic University (PolyU).

From Sunday afternoon, rioters gathering around PolyU have taken the campus as a stronghold to launch assaults on police officers using weapons including petrol bombs, bows and arrows, metal balls and bricks.

HK universities have become weapon factories, said a police officer when he briefed the media about recent violence on Hong Kong campuses.

A total of 3,900 petrol bombs were found in the Chinese University of Hong Kong (CUHK), and the number of petrol bombs found at PolyU is estimated to be larger, the police told reporters.

Three universities have reported to police that chemicals were stolen from their campuses amid intense clashes.

Among them, the CUHK has lost around 100 liters of concentrated nitric acid and concentrated sulphuric acid, said the police, warning a potential risk to public security.

Arranged by the police, over 20 injured people have received treatment, and authorities also allowed medical workers into PolyU. 

With the help of the police, dozens of principals, teachers and staff from relevant departments were arranged to persuade people on the campuses to leave.

According to the police, 600 people have left PolyU during the operations, with about 400 of them being adults.

Six police officers were injured in Monday's operation, said the police.

UN decries HK protesters' violence, calls for calm

The UN human rights office said on Tuesday that some Hong Kong demonstrators' resorting to extreme violence, including against police, was "deeply regrettable and cannot be condoned" and voiced concern that the situation could deteriorate further.

Spokesman Rupert Colville urged Hong Kong authorities to de-escalate the situation at the PolyU, and addressed the humanitarian situation of those inside "which is clearly deteriorating."