Violence flared up again in Hong Kong on Sunday, as black-shirted radicals firebombed an entrance to the High Court Building in Central, spraying graffiti on the outer walls of the courthouse.

The police said in a statement that rioters threw petrol bombs outside the High Court and the Court of Final Appeal adjacent to the finishing point of the procession in the evening. The exterior walls of the High Court were also spray-painted.

The cases were classified as "arson" and will be investigated by the Central District Investigation Team, the police said.

The Hong Kong Special Administrative Region government will not condone any acts of sabotage against the Judiciary or damage to the rule of law, the Department of Justice (DoJ) said in a statement, adding that arson is a serious offence and the maximum penalty is life imprisonment.

The arson not only disrupted social peace but also undermined Hong Kong's reputation of being governed by the rule of law, the DoJ said.

Besides, a group of violent protesters vandalized shops and a bank in Causeway Bay and Wan Chai. The police warned them not to do any illegal acts posing threat to public order and safety.

A public gathering was held at the Victoria Park on Sunday afternoon, followed by a procession expected to finish at Chater Road in the Central district. Some participants deviated from the approved route and occupied part of Gloucester Road and Des Voeux Road Central.

The police also asked all participants not to deviate from the approved route and advised them to disperse after the assembly.