At least nine people, aged between 17 and 59, were injured in the protests, according to the Hong Kong Information Services Department.
One man was in serious condition and one man in stable condition, the information department said. Four other men and three women have been discharged from hospital.
CNN teams witnessed riot police raising the black flag, which serves as a warning of tear gas, in several locations on Hong Kong island and across the bay in Kowloon during Sunday evening. Police clad in riot gear then fired tear gas canisters at the crowds in an attempt to disperse them.
Protesters initially congregated in Victoria Park for an “authorized” gathering. As they moved into surrounding areas, police said they were marching illegally and called on them to disperse.
Police said the demonstrators also set up barricades on Hennessy Road, blocking the thoroughfare and throwing petrol bombs at police officers.
A statement from police “condemned all violent acts by the protestors.”
“Residents of the area are advised to stay tuned to the latest situation and if necessary, keep their windows shut and stay indoors,” the statement said.
Meanwhile, thousands of activists continued their three-day sit-in at Hong Kong’s international airport.
The protest is intended to be peaceful and has not caused a major disruption. Demonstrators were handing out postcards explaining their demands. In reaction, China has issued a ban on staff from Cathay Pacific Airways who have participated in the demonstrations.
Hong Kong has plunged into chaos over a controversial bill that would allow extraditions from the city to mainland China.
The bill has been shelved, but the protests have shown no sign of easing. Instead, they’ve evolved to include a range of demands, including greater democracy and an inquiry into alleged police brutality.
Almost 600 people have been arrested so far, police said this week, for a range of offenses including “taking part in a riot,” unlawful assembly, assaulting police officers, resisting arrest and possession of offensive weapons.
On Thursday, the United States became the latest country to issue a travel warning for Hong Kong, urging travelers to exercise increased caution over the protests.
“Most have been peaceful, but some have turned confrontational or resulted in violent clashes,” the advisory said. “These demonstrations, which can take place with little or no notice, are likely to continue.”