Dozens of protesters in Hong Kong remain barricaded inside a besieged university campus for a fourth straight day as a violent standoff between pro-democracy demonstrators and police continues.

Protesters at Hong Kong Polytechnic University (PolyU) said on Wednesday some 50 remained after hundreds had fled worsening conditions and following official warnings that police could fire live rounds to clear the area.

Photos showed youths wandering the campus, with some preparing Molotov cocktails while others slept on a gym floor.

The standoff has been the most intense and prolonged in nearly six months of unrest that began over a now-shelved bill to allow extraditions to China, which revived fears that Beijing was cutting into the city’s freedoms.

Carrie Lam, the Beijing-backed Hong Kong chief executive, on Tuesday called for the protesters to surrender, adding that those over 18 would face rioting charges, but minors would not be arrested.

Others were medically evacuated on stretchers overnight, and on Wednesday before dawn police chased down and arrested around a dozen students making a break for it.

Police said that since the siege began around 800 people had left the campus as of late Tuesday night.

The United Nations human rights office was watching the university situation with “deepening concern”, spokesman Rupert Colville said in Geneva.

The violent standoff between demonstrators and police at PolyU rippled overseas, with the United Nations calling for a peaceful resolution to the siege, while the US senate passed new legislation supporting protesters’ demands.