A former liberal studies teacher was sentenced to nine months in prison on Monday for taking part in an illegal assembly at the start of the social unrest three years ago.

Eastern Court heard that Yeung Tsz-chun, 32, took part in the protest against the proposed changes to Hong Kong's extradition laws outside government headquarters in Admiralty on June 12, 2019.

In mitigation, the defence lawyer said his client had always understood the need to shoulder his legal responsibility and therefore chose to plead guilty at the first instance.

The lawyer went on to say that although Yeung appeared to have lived a normal life after the incident, he had been doxxed, his family members were harassed over the phone and he had quit his job and became a publisher.

He added that Yeung sustained an injury to his right eye during the protest and suffered permanent impairment.

But principal magistrate Ada Yim rejected his injury as a factor for mitigation, saying Yeung didn't leave the scene when the situation deteriorated even though he had claimed he was hoping to take part in a "peaceful assembly."

She added it's regrettable that anyone was injured in that sort of situation.

The magistrate also said Yeung had moved railings around with others, describing this as an obvious show of support to protesters which could lead to people ignoring police warnings and even violence.

She also said she believed his action was intended to prolong the illegal assembly.