Beating suspects upon their arrest appeared to be widespread, it added.

“Video footage from the Prince Edward MTR station last Saturday night show riot police launching indiscriminate attacks, without any apparent lawful excuse, and using pepper spray on passengers inside a train compartment or hitting them with batons, especially since the officers in question left the train carriage afterwards without making any arrests,” the association said.

It said members of the association who had assisted suspects had been obstructed at the police station, resulting in denial of timely legal assistance to them.

“Arrested people have also complained of abuses suffered during detention, many of whom required hospitalisation or other non-trivial medical treatments,” it said.

Police have previously said officers entered the station last Saturday at the request of the MTR Corp because protesters were vandalising the station, while others fought with a group of elderly passengers on the train.

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The force has denied allegations that entry into the station was unwarranted, and has also said it has not stopped protesters from meeting with lawyers.

Firing pepper balls at protesters from close range would also “not have the effect most expect”, the force said previously.

After the rally at Tamar Park had ended, some protesters made their way to Prince Edward and Wong Tai Sin, another spot that had recently seen several intense clashes.

At around 11.15pm, police intercepted and subdued two men in black tops on the concourse of Prince Edward station. One, who was wearing a gas mask, passed out during the incident.

Passers-by surrounded the officers, demanding to know why the man had been subdued and calling for no more harm to be done to him. As a volunteer paramedic tried to help the man, officers ordered the crowd to step back to make room.

With the situation tense and the crowd still shouting abuse, riot police entered the station from another entrance. But people tried to block them, resulting in officers using pepper spray and swinging their batons to disperse the crowd.

With the man passed out on the floor, officers moved him to the MTR control room a few metres away. Passers-by yelled at the police that they should not move an unconscious man.

Paramedics later arrived and took the man to hospital.

Into the early hours of Wednesday, a small crowd of protesters remained around Mong Kok Police Station, close to the MTR station.

The man’s father said at the Kwong Wah Hospital, where his 21-year-old son was being treated, that his son was conscious and able to talk. The young man sustained injuries to his head and face, the father added.

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Meanwhile, after a minor attempt to block Lung Cheung Road in Wong Tai Sin, some protesters got on a bus on route 42C heading to Lam Tin station at 9.10pm. About 15 minutes later, as the KMB double decker arrived in Kowloon Bay, several people tried to get off but were met by dozens of police.

Officers entered the bus and ordered some of the about 20 people on board to put their hands on their head. The double decker did not leave the bus stop.

Police then searched people’s belongings and took their ID card details.

Angry residents and commuters complained the investigation was inconvenient and made no sense. They shouted abuse at police.

Officers warned residents surrounding them not to shout loudly at night and to stop carrying out acts that constituted a breach of the peace.

More than two hours later, police and those on board had not left the bus, and police eventually arrested some of the passengers.