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Parts of the Hong Kong metro system ground to a halt after violence escalated between police and protesters as months of unrest in the former British territory continue.

Television footage showed officers beating protesters with their batons as people cowered behind umbrellas on the floor of the Prince Edward metro station.

Police said they arrested 40 people inside the station on suspicion of obstructing officers, unlawful assembly and criminal damage. Three stations stayed shut on Sunday.

Meanwhile, the train service to Hong Kong’s airport was suspended as pro-democracy demonstrators gathered there.

Protesters outside the British Consulate called on London to grant citizenship to people born in the former colony before its return to China.

Hong Kong has been the scene of increasingly tense anti-government protests for nearly three months.

They began in response to a proposed extradition law and have expanded to include other grievances and demands for more democracy and the resignation of the territory’s leader.

"The level of violence is rapidly escalating, and their illegal acts have no regard to the laws of Hong Kong," police said in a statement that condemned unlawful assembly of protesters in various districts.

But protesters said that the people of Hong Kong were caught in a humanitarian crisis, and appealed for international help in a statement on social media.

The post described the police crackdown at the Prince Edward station as a "terrorist attack".

"They beat up innocent people causing them serious wounds," protesters said in the post signed, "Hongkongers".

"Some passengers kneeled before them asking them to stop but...were beaten even harder."

At the airport, several hundred protesters gathered at the terminal on Chek Lap Kok island in the early afternoon and set up temporary barricades at a bus terminal.

A dozen blue-uniformed police officers in riot helmets formed a line across an entrance corridor to keep them out of the terminal.

Many carried umbrellas, a symbol of the protest movement.

The operator of the express train to the airport from central Hong Kong said service was suspended.​ MTR said trains into the city from the airport are still running.

Police issued a statement warning protesters were violating a court injunction against disrupting airport operations.

Hong Kong protests - In pictures 19 show all Hong Kong protests - In pictures 1/19 Riot police react after one of the protesters has thrown a bottle with flammable liquid EPA 2/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 3/19 Protesters with umbrellas and protective gear face off with riot police at Kowloon Bay AFP/Getty Images 4/19 Protesters in action against riot police during an anti-government march in Kwun Tong EPA 5/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 6/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 7/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 8/19 Protesters face off with riot police at Kowloon Bay in Hong Kong AFP/Getty Images 9/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 10/19 A demonstrator throws back a tear gas canister as they clash with riot police during a protest in Hong Kong REUTERS 11/19 Riot police fire tear gas to disperse anti-extradition bill protesters after a march to demand democracy and political reforms REUTERS 12/19 Protesters take part in an anti-government march in Kwun Tong EPA 13/19 Protesters stand on a wall during a face off with police after an anti-government march in Kwun Tong EPA 14/19 protester directs other protesters to safer grounds after clashing with the police after an anti-government march in Kwun Tong EPA 15/19 Protesters stand behind a makeshift barricade set up near the Ngau Tau Kok police station after an anti-government march in Kwun Tong EPA 16/19 A protester displays a Hong Kong Independence flag during an anti-government march in Kwun Tong EPA 17/19 Riot police clash with protesters during an anti-government march in Kwun Tong EPA 18/19 Protesters react after riot police fired tear gas during an anti-government march in Kwun Tong, Hong Kong EPA 19/19 Protesters in action against riot police during an anti-government march in Kwun Tong EPA 1/19 Riot police react after one of the protesters has thrown a bottle with flammable liquid EPA 2/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 3/19 Protesters with umbrellas and protective gear face off with riot police at Kowloon Bay AFP/Getty Images 4/19 Protesters in action against riot police during an anti-government march in Kwun Tong EPA 5/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 6/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 7/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 8/19 Protesters face off with riot police at Kowloon Bay in Hong Kong AFP/Getty Images 9/19 A protester throws back a tear gas canister during clashes with riot police at Kowloon Bay AFP/Getty Images 10/19 A demonstrator throws back a tear gas canister as they clash with riot police during a protest in Hong Kong REUTERS 11/19 Riot police fire tear gas to disperse anti-extradition bill protesters after a march to demand democracy and political reforms REUTERS 12/19 Protesters take part in an anti-government march in Kwun Tong EPA 13/19 Protesters stand on a wall during a face off with police after an anti-government march in Kwun Tong EPA 14/19 protester directs other protesters to safer grounds after clashing with the police after an anti-government march in Kwun Tong EPA 15/19 Protesters stand behind a makeshift barricade set up near the Ngau Tau Kok police station after an anti-government march in Kwun Tong EPA 16/19 A protester displays a Hong Kong Independence flag during an anti-government march in Kwun Tong EPA 17/19 Riot police clash with protesters during an anti-government march in Kwun Tong EPA 18/19 Protesters react after riot police fired tear gas during an anti-government march in Kwun Tong, Hong Kong EPA 19/19 Protesters in action against riot police during an anti-government march in Kwun Tong EPA

The protesters were mostly peaceful, but a government statement said some threw objects at police and airport employees.

A separate statement said iron poles, bricks and rocks were thrown on to the tracks of the airport train, “seriously obstructing” service.

About 200 demonstrators also congregated outside the British Consulate.

A saxophonist in dark glasses played God Save The Queen. Many wanted London to grant citizenship to people born in Hong Kong before 1997.

The British government refused that, instead giving people in the colony British National Overseas passports they could use to travel abroad but not settle in the United Kingdom.

“I hope the British Government can change its nationality law,” said protester Gary Law.

Police issued a statement warning that protesters were violating a court injunction against Sunday's demonstration. A police statement said officers were preparing to clear away the protesters.

The latest protests came on the fifth anniversary of China's decision to curtail democratic reforms and rule out universal suffrage in Hong Kong.

Hong Kong teachers' rally - In pictures 28 show all Hong Kong teachers' rally - In pictures 1/28 Protesters gather for a march organised by a pro-democracy group in Hong Kong AP 2/28 People shelter from the rain during the the 'Restore Tranquility to Our Homeland' demonstration against the extradition bill REUTERS 3/28 A pro-democracy protester ties a white-ribbon, symbolising the pure intentions of young protesters during a march organized by teachers in Hong Kong AP 4/28 Pro-democracy protesters stand in solidarity with young protesters during Saturday's rain-soaked march AP 5/28 Thousands of protesters take to Hong Kong's streets on Saturday REUTERS 6/28 A pro-democracy protester with his eye covered with a red-eyepatch in tribute to a women reported to have had an eye ruptured by a beanbag round fired by police during clashes AP 7/28 Protesters take part in the march titled 'Recover Hung Hom' in To Kwa Wan, Hong Kong EPA 8/28 A protester prepares to throw an egg at a pro-government lawmaker during Saturday's march AP 9/28 A pro-democracy protester gestures during rallies in Hong Kong REUTERS 10/28 A protester walks past anti-government graffiti on a wall in the city EPA 11/28 A protester waves a British colonial flag during Saturday's pro-democracy rally EPA 12/28 A protester displays a placard with an image of Hong Kong leader Carrie Lam AP 13/28 Children were among the participants in Saturday's peaceful rally REUTERS 14/28 Locals watch as thousands stream through the city REUTERS 15/28 Teachers and thousands of pro-democracy protesters took to the streets REUTERS 16/28 Family members offer drinks to protesters during the rally REUTERS 17/28 Protesters march through the streets of Hong Kong AP 18/28 A protestor covers his right eye during the protests REUTERS 19/28 Protesters march through the streets of Hong Kong AFP/Getty Images 20/28 Protesters march through the streets of Hong Kong REUTERS 21/28 Protesters march through the streets of Hong Kong REUTERS 22/28 Protestors voicing support for democracy in Hong Kong gather at a demonstration outside the State Library of Victoria in Melbourne, Australia EPA 23/28 Protestors voicing support for democracy in Hong Kong gather at a demonstration outside the State Library of Victoria in Melbourne, Australia EPA 24/28 Pro-democracy demonstrators march in support of the Hong Kong democracy movement outside the State Library of Victoria in Melbourne, Australia EPA 25/28 A pro-democracy demonstrator holding a Taiwanese flag clashes with pro-China supporters during a demonstration near Town Hall in Sydney EPA 26/28 A Pro-China activist marches on the streets of Sydney on August 17 AFP/Getty Images 27/28 Pro-China activists hold up placards and China with Hong Kong flags while marching on the streets of Sydney AFP/Getty Images 28/28 Pro-China activists take to the streets of Sydney AFP/Getty Images 1/28 Protesters gather for a march organised by a pro-democracy group in Hong Kong AP 2/28 People shelter from the rain during the the 'Restore Tranquility to Our Homeland' demonstration against the extradition bill REUTERS 3/28 A pro-democracy protester ties a white-ribbon, symbolising the pure intentions of young protesters during a march organized by teachers in Hong Kong AP 4/28 Pro-democracy protesters stand in solidarity with young protesters during Saturday's rain-soaked march AP 5/28 Thousands of protesters take to Hong Kong's streets on Saturday REUTERS 6/28 A pro-democracy protester with his eye covered with a red-eyepatch in tribute to a women reported to have had an eye ruptured by a beanbag round fired by police during clashes AP 7/28 Protesters take part in the march titled 'Recover Hung Hom' in To Kwa Wan, Hong Kong EPA 8/28 A protester prepares to throw an egg at a pro-government lawmaker during Saturday's march AP 9/28 A pro-democracy protester gestures during rallies in Hong Kong REUTERS 10/28 A protester walks past anti-government graffiti on a wall in the city EPA 11/28 A protester waves a British colonial flag during Saturday's pro-democracy rally EPA 12/28 A protester displays a placard with an image of Hong Kong leader Carrie Lam AP 13/28 Children were among the participants in Saturday's peaceful rally REUTERS 14/28 Locals watch as thousands stream through the city REUTERS 15/28 Teachers and thousands of pro-democracy protesters took to the streets REUTERS 16/28 Family members offer drinks to protesters during the rally REUTERS 17/28 Protesters march through the streets of Hong Kong AP 18/28 A protestor covers his right eye during the protests REUTERS 19/28 Protesters march through the streets of Hong Kong AFP/Getty Images 20/28 Protesters march through the streets of Hong Kong REUTERS 21/28 Protesters march through the streets of Hong Kong REUTERS 22/28 Protestors voicing support for democracy in Hong Kong gather at a demonstration outside the State Library of Victoria in Melbourne, Australia EPA 23/28 Protestors voicing support for democracy in Hong Kong gather at a demonstration outside the State Library of Victoria in Melbourne, Australia EPA 24/28 Pro-democracy demonstrators march in support of the Hong Kong democracy movement outside the State Library of Victoria in Melbourne, Australia EPA 25/28 A pro-democracy demonstrator holding a Taiwanese flag clashes with pro-China supporters during a demonstration near Town Hall in Sydney EPA 26/28 A Pro-China activist marches on the streets of Sydney on August 17 AFP/Getty Images 27/28 Pro-China activists hold up placards and China with Hong Kong flags while marching on the streets of Sydney AFP/Getty Images 28/28 Pro-China activists take to the streets of Sydney AFP/Getty Images

The unrest began in mid-June, fuelled by anger over a now-suspended extradition bill that would have allowed people in the city to be sent to mainland China for trial in courts controlled by the Communist Party.

But the turmoil has evolved over 13 weeks to become a widespread demand for greater democracy in a territory that many residents believe is increasingly being controlled by Beijing.

Hong Kong returned to China under a "one country, two systems" formula that allows it freedoms not enjoyed on the mainland, such as the right to protest and an independent legal system.

The unrest poses the gravest challenge to Chinese President Xi Jinping since he took power in 2012, with his government keen to end the protests before the 70th anniversary of the founding of the People's Republic of China on Oct 1.

Beijing has also accused foreign powers, particularly the United States and Britain, of fomenting the demonstrations.