Videos showing the man being apprehended by the police in the station have been widely shared with protest groups saying it is evidence of police brutality.
Two men were in a stable condition in hospital on September 4 after Hong Kong police clashed with pro-democracy protestors overnight, as unrest that has gripped the Chinese-ruled city for months showed little sign of abating.
Riot police fired beanbag guns and used pepper spray — both anti-riot weapons — to clear demonstrators from outside the Mong Kok police station and in Prince Edward metro station, with one man taken out on a stretcher with an oxygen mask over his face, television footage showed.
Videos showing the man being apprehended by the police in the station have been widely shared on social media with protest groups and activists saying it is evidence of the police brutality they say is widespread and needs to be investigated.
The police, who have repeatedly denied using excessive force, did not immediately respond to a request for comment. Hong Kong police are due to hold a news conference at 4 p.m. local time.
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Three men, aged between 21 and 42, were taken to Kwong Wa Hospital late on Tuesday, a hospital authority spokeswoman said.
Two, including the man stretchered out of Prince Edward station, were in a stable condition and one had been discharged, she said.
Hundreds of thousands of people have taken to the streets since mid-June in sometimes violent protests calling for greater democracy in the former British colony, posing a direct challenge to the central Chinese government in Beijing.
Police have arrested more than 1,100 people since the protests started.
Metro stations have often become the front line in battles between masked protesters and police, deepening the biggest political crisis in the Asian financial hub since its handover from Britain to China in 1997.
Pressure on Carrie Lam
The unrest is piling pressure on Hong Kong's leader Carrie Lam, who told a group of business people last week that she had ”very, very limited” ability to end the crisis and it had been elevated “to a national level”, a reference to the leadership in Beijing. Her comments in an audio recording of the meeting were reported by Reuters on Tuesday.
Ms. Lam also said in the recording that she would step down if she had a choice, fuelling protestors' complaints that the partial autonomy granted to Hong Kong in 1997 under a “one country, two systems” agreement is being slowly eroded by Beijing.
Beijing denies that and says it is committed to the formula.
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Ms. Lam on Tuesday said she had never discussed resignation with Beijing and believed her government could solve the crisis without Beijing's help.
China denies it is meddling in Hong Kong's affairs but warned again on Tuesday that it would not sit idly by if the unrest threatened Chinese security and sovereignty.
Protests that began over a now-suspended extradition bill have evolved into a push for greater democracy, including the right to elect its own leaders. Beijing has said giving Hong Kong universal suffrage is out of the question.
China has denounced the protests and warned about the impact on Hong Kong's economy. China has also accused Western countries of stirring up the unrest.
With protestors and Ms. Lam's government at an impasse there are concerns the city's economy could go into a tailspin, with signs already that money is moving out to other financial centres, including Singapore.
Hong Kong's private sector activity declined at the fastest pace in more than a decade in August as the protests and an escalating trade war between China and the United States hit demand, data showed on Wednesday.