Protesters have torched vehicles and buildings, hurled petrol bombs at police stations and trains and vandalised prime shopping malls during the past week in some of the worst violence seen in more than five months of unrest.

Hundreds of protesters on Thursday occupied roads in the Central business district, home to some of the world's most expensive real estate, during their lunch hour.

Many were dressed in office attire and wore the now-banned face masks as they marched down a major thoroughfare that connects luxury shopping malls and glittering skyscrapers.

Hundreds of protesters also set up barricades near a popular shopping mall in the eastern part of Hong Kong island, dragging traffic cones and orange gates to block roads.

Some held up signs reading "Liberate Hong Kong, revolution of our time" and "Stand with Hong Kong", slogans that have become rallying calls for the protest movement.

Across the harbour, black-clad protesters and university students maintained their blockades of major roads, including the entrance to the Cross-Harbour Tunnel that links Hong Kong island to the Kowloon area, and a highway between Kowloon and the rural New Territories.

Police fired tear gas near the tunnel early on Thursday to try to clear the protesters. Roads were strewn with bricks and other debris, leading to widespread traffic jams.

Thousands of students barricaded themselves inside campuses at several universities overnight, preparing stockpiles of food, bricks, petrol bombs and other make-do weapons as they hunkered down.

Dozens of riot police gathered outside several universities early on Thursday as students fortified their positions with metal poles, bricks and chairs.

At Hong Kong's Polytechnic University, near the Kowloon entrance of the Cross Harbour tunnel, hundreds of students wearing gas masks readied for confrontations with police.

Boxes of petrol bombs were placed at vantage points overlooking roads including the tunnel, which has been blocked since Wednesday evening.

Students also poured litres of cooking oil across the ground in an attempt to thwart police should they try to enter.

Masses of commuters queued at metro stations across the city after some rail services were suspended and roads closed.

The demonstrations were initially spurred by what many residents see as the stifling of freedoms guaranteed under a "one country, two systems" formula put in place when the territory returned to Chinese rule in 1997.

In particular, people were alarmed by a proposed law, now dropped, that would have seen criminal suspects liable to prosecution in mainland courts.

Anger grew about what many see as police brutality as the protests intensified.

China denies interfering and has blamed Western countries for stirring up trouble. Police deny using excessive force.

Police said on Wednesday violence had reached a "very dangerous and even deadly level".

Authorities said 64 people were injured during Wednesday's clashes, which left two men in critical condition. There were no further details about their injuries.

Police said a man had died after falling from a height on Wednesday but gave no further details.

Hong Kong's embattled leader, Carrie Lam, met senior officials late on Wednesday, media reported, amid speculation of new emergency measures to deal with the crisis.

Australian Associated Press