A Hong Kong student who sustained head injuries when he fell during clashes with police died on Friday, triggering a fresh wave of outrage from the pro-democracy movement and fears of more violent unrest.

Alex Chow, a 22-year-old computer science undergraduate, was certified dead on Friday morning, according to the hospital where he was being treated.

Chow was taken unconscious to hospital early on Monday morning following late-night clashes between police and protesters in a middle-class district.

He was found lying unconscious in a pool of blood inside a car park that police had fired tear gas towards after protesters hurled objects from the building.

Although the precise chain of events leading to Chow's fall are unclear and disputed, his death marks the first student fatality during five months of demonstrations challenging China's rule of the city.

Chow's case has been embraced by the protest movement, which is demanding Beijing loosen its control over the city and allow more democratic freedoms.

"Today we mourn the loss of the freedom fighter in HK," Joshua Wong, a prominent pro-democracy campaigner, said on Twitter.

"We will not leave anyone behind - what we start together, we finish together. Given the losses suffered by HK society in the past month, the gov must pay the price."

Online forums used by the largely anonymous and leaderless protest movement also quickly filled up with calls for city-wide vigils on Friday night to mourn Chow's death.

Chow was a student at Hong Kong University of Science and Technology.

The college held its graduation ceremony Friday morning, and university head Wei Shyy paused the proceedings to announce Chow's death.

Fellow students had been holding a vigil for Chow as doctors battled to save his life.

Sources told AFP doctors had performed two operations in a bid to reduce swelling in his brain.

Chow fell from a ledge of the multi-storey car park in Tseung Kwan O, a residential area where clashes between protesters and police have occurred regularly.

Police officials have acknowledged that tear gas had been used to disperse protesters near the car park where Chow fell on Sunday night.

But they denied any wrongdoing, saying their use of tear gas was justified.

They have also denied allegations of interfering with rescuers treating the student, or blocking the ambulance that took him to hospital.

Millions of people have taken to the streets since the unrest began in June, with protesters calling for greater police accountability as well as democratic freedoms.

China has run the city under a special "one country, two systems" model, allowing Hong Kong liberties not seen on the mainland, since its handover from the British in 1997.

But public anger has been building for years over fears that Beijing is eroding those freedoms, especially since President Xi Jinping came to power.

Protesters have issued a list of demands, including fully free elections to choose the city's leader and an investigation into alleged abuses by police.