Image copyright Reuters Image caption Students at Mr Chow's college, the University of Science and Technology, mark Alex Chow's death after his death was announced during a graduation ceremony

A Hong Kong student has died after falling during protests on Monday - sparking impromptu protests and vigils from pro-democracy activists.

Alex Chow fell from the ledge of a car park during a police operation to clear the area on Monday morning.

The exact circumstances of his fall are unclear, but reports say he was trying to get away from tear gas.

The 22-year-old's death came after another week of political violence in Hong Kong.

On Wednesday a pro-Beijing lawmaker was stabbed in the street by a person pretending to be a supporter.

Mr Chow had been in a coma since Monday and was certified dead early on Friday, according to the hospital he was treated in.

Fellow students at the city's University of Science and Technology called for an investigation into how Mr Chow fell, and why it took paramedics nearly 20 minutes to arrive. Amnesty International echoed that call.

At the time of his fall, dozens of riot police were approaching, firing tear gas into the car park and surrounding areas to clear away protesters.

Image copyright Reuters Image caption Students attended a ceremony to pay tribute to the 22-year-old at Hong Kong's University of Science and Technology

Police said they used tear gas to disperse protesters near the site but denied any wrongdoing, saying their actions had been justified.

But Mr Chow's death is expected to fuel more anger at the police, who are already under pressure as Hong Kong faces its worst crisis in decades.

"Today we mourn the loss of the freedom fighter in HK," Joshua Wong, a pro-democracy campaigner, said on Twitter. "We will not leave anyone behind - what we start together, we finish together."

Protesters gathered across the city, taking part in mourning events. Thousands left flowers at the spot in which he fell at the car park. Some people sang hymns.

Image copyright EPA Image caption Thousands have paid tribute to Mr Chow, leaving flowers and candles at the site where he fell

A 23-year-old student, who said she attended the same university as Mr Chow, told AFP news agency: "I think there should be an independent inquiry commission to investigate his death and other incidents that happened during the movement."

Other people gathered in the shopping district of Causeway Bay, lining the streets in silence.

At Hong Kong's University of Science and Technology, dozens of protesters vandalised a Starbucks, Bank of China and at least three cafeterias on site, the South China Morning Post reports.

They wrote "condemn police brutality" on the walls of the Starbucks store.

Protesters are calling for more demonstrations this weekend.

Media playback is unsupported on your device Media caption How Hong Kong got trapped in a cycle of violence

Why are there protests in Hong Kong?

Hong Kong is part of China but as a former British colony it has some autonomy and people have more rights.

The protests started in June against plans to allow extradition to the mainland - which many feared would undermine the city's freedoms.

The bill was withdrawn in September but demonstrations continued and now call for full democracy and an inquiry into police behaviour.

Clashes between police and activists have become increasingly violent and in October the city banned all face masks.