Students stand near a charred vehicle during a face-off with riot police in Hong Kong on Tuesday. AP

Police and protesters battled outside university campuses and several thousand demonstrators blocked roads as they took over a central business district at lunchtime on Tuesday in another day of protest in Hong Kong.

The clashes followed an especially violent day in Hong Kong's five months of anti-government demonstrations, in which police shot one protester and a man was set on fire.

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Protesters littered streets with bricks and disrupted train service during the morning rush hour on Tuesday. Commuter train passengers were escorted along the tracks, and subways were shut because of disruptions.

A man is detained after police fired tear gas at the Chinese University of Hong Kong on Tuesday. AFP

Police used tear gas in faceoffs with protesters in and around universities, where classes were canceled.

Following a standoff outside Chinese University, scores of officers charged onto the campus after firing tear gas, arresting student protesters who tried to block their way with makeshift barricades, including a burning car.

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A few thousand protesters took over several blocks of the central business district at lunchtime. The demonstrators chanted "Five demands, not one less" holding up one hand with five outstretched fingers. Their demands include democratic changes and investigation of police treatment of protesters.

Traffic was blocked on two major roads by the crowds, with half a dozen of Hong Kong's famous trams lined up unable to move. The words "Join Us" were spray painted on the front window of a halted double-decker bus abandoned by the driver and passengers and one of its windows was broken.

Office workers and pro-democracy protesters gather during a demonstration in Hong Kong on Tuesday. AFP

Office workers filled the sidewalks and overhead walkways to watch the action, with some joining the protesters in chanting.

One 24-year-old man, who would not give his name, said he was there to support the protesters and accused the police of using excessive force, a common complaint among the city's 7.4 million people.

Police fired tear gas to disperse protesters and onlookers who were hurling abuse at the officers. At least one person was injured when he was struck on the head by a tear gas canister. But protesters returned by evening and were again blocking roads with bricks and commandeered buses.

Police (left) fire tear gas towards protesters (back) at the Chinese University of Hong Kong on Tuesday. AFP

Recent weeks have been marked by escalating vandalism against shops linked to mainland China and train stations, and assaults by both protesters and pro-Beijing supporters.

Hong Kong leader Carrie Lam, speaking to news media after a weekly meeting with advisers, called the blocking of the morning commute "a very selfish act."

"People from different sectors in society are holding fast to their positions and refusing to concede to violence or other radical actions," she said. "I hereby express my gratitude to those who are still going to work and school today."

On Monday, a police officer drew his gun during a struggle with protesters, shooting one in the abdomen. In another neighborhood, a 57-year-old man was set on fire after an apparent argument.

Both remained hospitalised Tuesday, the shot protester in serious condition and the man who was burned in critical condition, the Hospital Authority said.

Video of another incident showed a policeman on a motorcycle riding through a group of protesters in an apparent attempt to disperse them.

Police say those events are being investigated but defend the officers' actions as necessary for their own safety.

Police look for pro-democracy protesters along a road during a protest in Central in Hong Kong on Tuesday. AFP

Police spokesman Kong Wing-cheung said the burning had been registered as a case of attempted murder and called on the public to provide information about the assailant.

"Hong Kong's rule of law has been pushed to the brink of total collapse," Kong said, calling those who defend or maintain ties with violent protesters "accomplices."

In Beijing, foreign ministry spokesman Geng Shuang lambasted the U.S. and Britain over statements of concern over the spike in violence.

"The United States and Britain pretend to be fair on this incident, but it only reveals how they confuse right and wrong and how hypocritical they are. And their verbal justice once again exposes their double standards and ulterior motives," Geng said at a daily briefing.

Associated Press