Demonstrators defied a ban on protest by the authorities in the territory.
Hong Kong has witnessed some of its worst violence in weeks as protesters set fire to metro stations and attacked the offices of the Xinhua news agency, China's state media outlet.
As the demonstrations enter their 22nd week, police fired tear gas at thousands gathered in a park downtown, while small groups of protesters rampaged through the city's central business district.
As police used water canon, protesters threw petrol bombs and hauled telephone booths onto burning barricades. As well as Xinhua, branches of Starbucks - the U.S. coffee giant - were vandalised.
An employee at the news agency said that the front doors of the office had been smashed and paint and petrol bombs thrown into the lobby.
Protesters say they're angry at China for meddling with the freedoms Hong Kongers still enjoy, despite the territory being handed back to China by the UK in 1997..
China says it has not interfered with Hong Kong's special freedoms, a legacy of British rule, and called the protesters "violent mobs" and "criminals".
It has sent soldiers and military hardware to a nearby part of the mainland, possibly to move them into the city at a later stage.
It was announced this week that Hong Kong had gone into recession after weeks of protests deter tourists and hit the city's restaurant and nightlife trade.