Protesters in Hong Kong trampled a Chinese flag, vandalised a subway station and set a fire across a wide street on Sunday, as pro-democracy demonstrations took a violent turn once again.

The rail station was closed to passengers after protesters smashed it. Meanwhile, police stormed into a shopping mall in Shatin and fired tear gas in the district's New Territories town to disperse the anti-government protesters who had trashed fittings at the railway station.

The day's activities started peacefully as young protesters, many wearing masks to disguise their identity, filled the open area of a Hong Kong shopping mall and folded paper “origami” cranes in the latest twist in a pro-democracy movement that has stretched into a fourth month.

They chanted slogans and sang a song that has become their anthem, backed by a small group playing on woodwind and brass instruments through their masks. Many lined the balustrades of the three higher floors overlooking where others gathered in the wide space below.

Some put a Chinese flag on the floor and took turns running over it, before defacing it and putting it in a dumpster outside, which they then pushed into a nearby river.

One group later attacked the Shatin subway station, which is connected to the mall. They jumped up to smash overhead surveillance cameras, used hammers to knock ticket sensors off gates and spray-painted and broke the screens of ticket machines, using umbrellas to shield their identities.

Riot police arrived following the attack and guarded the station after it was closed, with a metal grill pulled down to block entry.

Protesters then built a barricade across a street near the mall, piled what appeared to be brown palm fronds on top and set them on fire.

Police fired tear gas as they tried to advance on the protesters, who had retreated before taking a position behind a wall of umbrellas that those in the front held.

The latest gathering came after a night of violent clashes in which the police used tear gas and rubber rounds against protesters who threw gasoline bombs toward them and set fires in streets.

The protests generally begin peacefully, but often degenerate into confrontations that hard-line protesters say is needed to get the government's attention.

Hong Kong's leader, Chief Executive Carrie Lam, has agreed to withdraw an extradition bill that first sparked the protests in June. But the anti-government protesters are pressing other demands, including fully democratic elections in the semi-autonomous Chinese city and an independent investigation of complaints about police violence during earlier demonstrations.

Protesters say Beijing and Lam's government are eroding the “high degree of autonomy” and Western-style civil liberties promised to the former British colony when it was returned to China in 1997.

The unending protests are an embarrassment for China's Communist Party ahead of October 1 celebrations of its 70th anniversary in power. Hong Kong's government has cancelled a fireworks display that day, citing concern for public safety.