"Police officers are conducting an operation to clean the territories in the eastern Hong Kong island [of protesters], while some of the protesters have set fires in various locations," the police said in a statement.
According to the police, protesters in the Sheung Wan area have set fire to a cart loaded with various items and pushed it towards a police cordon.
The statement also added that it condemned escalation of violence by protesters and called for calm.
Earlier on Sunday, the police detained fourteen people for taking part in unauthorised protests, including the 39-year-old organiser of the rally, Max Chung Kin-ping.
The rally began at 15:00 local time (07:00 GMT). Later in the day, some protesters went off the prescribed route, blocking central streets. At 18:05 local time, the police warned the demonstrators that tear gas could be fired. An hour later the police raised a red flag, which signals that the police will use force to disperse the crowd. Clashes between protesters and police officers are ongoing after the use of tear gas.
Mass rallies erupted in Hong Kong in early June as the authorities were considering to adopt a bill that would allow the autonomous Chinese city to extradite suspects to jurisdictions with which it did not have an extradition agreement, including mainland China. Pressured by the protests, the government indefinitely suspended the bill, while Hong Kong Chief Executive Carrie Lam issued a public apology. However, the protesters still demand the complete withdrawal of the extradition amendments.