A Hong Kong lawyer was badly beaten by several people clad in black following an argument just a few streets away from protests over Beijing’s proposed national security law for the city on Sunday.

The head of the Law Society confirmed the victim was a member and condemned the assault, saying that if the violence was over differing political stances, the state of affairs in Hong Kong was “pathetic”.

Legal sources identified the victim as Chan Tze-chin, who attended a Legislative Council session in his personal capacity last November to support a controversial law banning the use of masks to hide one’s identity. He is a partner at Cheung and Liu Solicitors and has both civil and criminal practices, and is the convenor of the Law Society’s swimming team.

According to police, the 40-year-old man got into an argument with dozens of people erecting roadblocks on Lee Garden Road in Causeway Bay at about 3.30pm. Several members of the group began hitting him with umbrellas and chased him when he tried to flee.

At one point, five “rioters” opened their umbrellas in an attempt to hide what their accomplices were doing, police said.

The attack was caught on video and circulated widely on social media, showing the victim on the ground as the masked men rain down blows to his head and torso near the head office of the Po Leung Kuk charity group on Leighton Road. As the assailants leave, he struggles to his feet, his face bloodied, and stumbles away.

The victim was sent to Ruttonjee Hospital with injuries to his head, arm and back, according to police. Ten suspects, aged between 20 and 40, were involved in the assault, the force said.

Law Society president Melissa Kaye Pang said the organisation was “very distressed and angry” over the attack.

“Wounding is a serious crime. Violence is violence no matter what principles lie behind it. It is absolutely unacceptable,” Pang said. “If someone responded with violence immediately because of different political stances … if this is the ‘freedom’ that everyone is fighting for, this will be what is pathetic about Hong Kong.”

Separately, police pointed to another video clip circulating on social media that showed several protesters hitting a woman with hard objects after she tried to remove roadblocks. When she fell down, more people joined in the attack. Video shows the assault took place at the junction of Hing Fat Street and Lau Li Street in Causeway Bay. Police are appealing for her to come forward.