Police gave no details of the two left in critical condition.

The pro-democracy protesters are campaigning against what they see as Chinese meddling with Hong Kong's promised freedoms.

China denies doing so and has blamed Western countries for stirring up trouble in the former British colony.

More demonstrations are planned this week as the protesters keep up pressure for demands such as an independent inquiry into police behaviour and for universal suffrage.

Protesters have circulated plans on social media to mark Guy Fawkes Day on November 5 by putting on now banned face masks in areas around Hong Kong.

Many people taking to the streets in recent weeks have worn the white smiling Guy Fawkes masks, made popular by anti-establishment hackers and by the film V for Vendetta.

The masks have also become common at protests globally, including in Britain and across the United States.

Hong Kong's embattled leader Carrie Lam last month invoked colonial era emergency powers for the first time in more than 50 years, banning face masks in a move to quell the protests.

Protesters have largely ignored the ruling and worn masks.

The protests have divided society in Hong Kong, and undermined its economy, with the police coming in for particular scrutiny.

"Civil servants are ashamed of the crimes committed by the Hong Kong police force and the dictatorship of the Hong Kong government," said the Citizens' Press Conference, a pro-democracy group that planned a Monday night discussion on the clashes.

Chinese state media called for a "tougher line" against the protesters who vandalised state-run Xinhua news agency and other buildings, saying the violence damaged the city's rule of law.

Protesters smashed up and torched Xinhua's office on Saturday in some of the worst violence in weeks, setting fire to metro stations and vandalising buildings, including an outlet of US coffee chain Starbucks.

Mainland businesses, including banks or companies seen as supportive of China's ruling Communist Party, have been targeted by protesters angered by China's perceived undermining of Hong Kong's freedoms since it returned to Chinese rule in 1997.

Police condemned the "rioters' violent and vandalistic acts" and vowed measures to uphold public safety.

Tear gas accidentally aimed at a fireman on Saturday had been intended to disperse rioters, the police said.

"There was misunderstanding in the verbal communication between both sides," they said on Monday. "The matter was tackled and resolved at the scene."

Reuters