Protesters have vandalised the Hong Kong office of China’s official Xinhua news agency for the first time in months of anti-government demonstrations, smashing windows and doors.

Local media showed scenes of a fire in the lobby of Xinhua’s office in Wan Chai district, broken windows and graffiti sprayed on a wall. It was unclear if there were people in the building.

Protesters have been targeting Chinese banks and businesses perceived to have links with Beijing as anger grows over what protesters say are infringments on freedoms guaranteed when Britain returned Hong Kong to China in 1997.

Police fired teargas rounds and used a water cannon to break up a rally on Saturday after some protesters threw petrol bombs at them.

Hong Kong police use water cannon as protesters throw petrol bombs – video, time 1:35

Scores occupied streets in the upmarket Causeway Bay shopping area, chanting pro-democracy slogans. Riot police conducted searches and issued warnings the rally was illegal and that protesters were violating a government ban on masks.

Teargas was also fired to disperse a huge crowd in nearby Victoria Park after protesters unearthed a goalpost from a football pitch and metal railings to block the park’s entrance. Several protesters were detained.

Others quickly regrouped in the vicinity, using plastic cones and metal railings to block roads in preparation for possible further clashes with police.

The anti-government protests began in early June over a plan, now shelved, to allow extraditions to mainland China. They have since morphed into a movement making other demands, including direct elections for Hong Kong’s leaders.

Saturday’s protests marked the 22nd weekend in a row of increasingly violent unrest that has damaged the reputation of one of the world’s top financial hubs. The city recently slipped into recession for the first time in a decade.

More than 3,000 people have been detained in the protests. The civil disobedience has posed a significant challenge to Beijing, which promised on Friday to prevent foreign powers from sowing acts of “separatism, subversion, infiltration and sabotage” in Hong Kong.

In a Communist party document released after its central committee meeting, Beijing said it would “establish and strengthen a legal system and enforcement mechanism” to safeguard national security in Hong Kong.

Hong Kong, which has a separate legal system from mainland China, has tried to enact anti-subversion legislation before, only to shelve the measure after formidable public opposition. Beijing may be indicating that it is preparing to take matters into its own hands by having the National People’s Congress issue a legal interpretation forcing its enactment.