"We will be outraged if there is no acceptable explanation offered," he said.
Hong Kong police have faced widespread criticism for use of tear gas deemed excessive since protests began in June in response to a proposed extradition bill that would have allowed criminal suspects in Hong Kong to be tried by courts in mainland China.
Since then, police have fired close to 6,000 tear gas canisters in the densely packed streets of Hong Kong, affecting millions of residents. Anger at this police response has sparked a demand for an independent investigation and has helped transform what started as single-issue marches into a wide-ranging antigovernment movement.
Several young protesters died by suicide this summer, leaving notes about despair over their failure to win concessions from the government. Unverified rumors have also swirled for months surrounding extrajudicial killings of protesters and subsequent cover-ups by police.
In June, activist Marco Leung Ling-kit fell to his death after hanging a banner opposing the extradition bill from the rooftop of a downtown shopping mall. Chow's death, however, is the first linked to a clearance operation by riot police.
At a news conference Friday afternoon, police spokeswoman Sze Yuk-sim said officers had entered the parking lot before Chow's fall but left long before it happened. She warned people about attending "unlawful assemblies," referring to impromptu memorial services for Chow that sprang up across the city throughout the day.
In Hong Kong, gatherings without a "letter of no objection" from police are deemed illegal, so participants can be arrested. More than 3,000 people have been arrested in the last five months of protests, more than a third of whom are students.
In Beijing, Ministry of Foreign Affairs spokesman Geng Shuang declined to comment on Chow's death. "At present, the most urgent thing for Hong Kong is to stop violence and restore order," he said.
As night fell, thousands of mourners gathered at the parking lot where Chow fell. A line of people dressed in black and wearing masks snaked through all three levels of the garage. They brought white flowers and candles, waiting quietly to lay their tributes at the spot where he hit the ground.
"When I heard the news on the bus this morning, I started crying," said Tiffany, 24, as she lighted a row of candles. Like other participants in the memorial, she asked not to disclose her full name for protection from authorities. "As a Hong Konger, I can't accept this happening. ... This isn't supposed to happen here."