Protests in Hong Kong took a violent turn Monday, heightening an already volatile situation days after a group of pro-democracy lawmakers was arrested in the city.

In one incident, a Hong Kong protester is in critical condition after being shot by police on Monday morning local time. A hospital press officer who spoke to CNBC confirmed that the 21-year-old man suffered a gunshot wound.

The police spokesperson said the officer involved in the shooting fired three times. That incident occurred at around 7 a.m. local time outside the Sai Wan Ho Mass Transit Railway station.

In an unrelated incident, police said "rioters" poured "flammable liquid" on a man and set him on fire. Speaking at an afternoon press conference, Hong Kong police said the incident occurred at the Ma On Shan MTR station and the man is in critical condition at a hospital.

Police said they are investigating the fire attack and made no mention of arrests.

Disturbing and graphic videos of those two incidents and other violence circulated widely on social media Monday.

In the last week, authorities have arrested a total of 266 people between the ages of 11 and 74, the Hong Kong police said Monday.

On Sunday, protesters blocked roads and vandalized shopping malls across Hong Kong's New Territories and Kowloon Peninsula during another weekend of anti-government demonstrations.