A screenshot from a video circulating on social media shows a police officer firing shots toward protesters—hitting at least one at close range—at an intersection in eastern Hong Kong island. Photo: cupid producer via Reuters

HONG KONG—Police fired gunshots at protesters early Monday, sending one man to the hospital where he underwent emergency surgery, as the city’s start-of-week commute descended into turmoil after demonstrators tried to block roads and delay trains.

In footage circulating on social media, a police officer was seen firing three shots toward protesters—hitting at least one at close range—at an intersection in eastern Hong Kong island. A 21-year-old man underwent an operation at a nearby hospital, according to the Hospital Authority, which declined to provide additional details on his condition.

Monday’s clashes created confusion for the city’s commuters who were heading to work or school. Many were left stranded at subway stations, while several universities canceled classes for the day.

Protests, now in their six month, have shown no signs of abating even after the government withdrew an extradition bill that initially sparked unrest. Weekend clashes are now commonplace as many young black-clad and masked protesters focus their anger on police, who they accuse of using excessive force to put down protests.

The shooting is likely to put more pressure on the government to consider protesters’ demands for a new independent investigation into police conduct. It is also likely to deepen the divide between Hong Kong residents and the government.

What started off as a demonstration against a controversial extradition bill has become a series of massive protests with broad political demands. Here is why so many Hong Kongers keep taking to the streets in a leaderless movement and whether their goals can be achieved. Photo: Thomas Peter/Reuters

The scuffles on Monday morning followed another weekend of clashes between police and protesters across town. In many districts across the city, residents were seen coming outside to shout abuse at police as they fired tear gas and made arrests.

Peaceful vigils were also held over the weekend for a student who died Friday morning from head injuries sustained in a fall from a parking garage in the early hours of last Monday while police were using tear gas in a nearby dispersal operation.

Monday’s shooting was the third live-fire incident that injured protesters since the demonstrations began. On Oct. 1, an 18-year-old student was shot in the chest by a riot police officer in the western New Territories while he was trying to attack another officer. Three days later, a 14-year-old boy was shot in the left thigh by a plainclothes officer who was surrounded by protesters.

At the scene of Monday’s shooting, a uniformed policeman drew his pistol with one hand and grabbed a protester with the other, the online video showed. Another black-clad protester approached the policeman and the officer fired at him. The shot protester fell to the ground immediately, clutching the right side of his stomach. The officer then fired two more rounds in the direction of other protesters. Hospital Authority officials didn’t comment on whether there was a second injured person.

After dawn Monday, protesters set up barricades at several key transportation interchanges around the city, including two road tunnels, according to police. More than a dozen locations were targeted for barricading by protesters, according to footage circulating online and posters in Telegram chatrooms where many protesters receive information about rallies.

A policeman walks under a cordon at the site where a police officer shot at pro-democracy protesters in Hong Kong on Monday. Photo: anthony wallace/Agence France-Presse/Getty Images

In some areas, police responded by firing tear gas. Some commuters got caught in the middle in the district of Tseung Kwan O, where the student died last week after falling from the parking garage. The affected commuters were treated by people appearing to be first aid workers who were carrying water and saline solution.

The Hong Kong Police Force said it opened fire early Monday morning in the Eastern District of Hong Kong island without providing details. Earlier, police issued a statement calling on protesters to stop obstructing traffic.

—Rachel Yeo and Joyu Wang contributed to this article.

Write to Eun-Young Jeong at Eun-Young.Jeong@wsj.com and Joanne Chiu at joanne.chiu@wsj.com

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