The director of the Hong Kong Social Workers' General Union, Hui Lai-ming, has been acquitted of obstructing the police at a Hong Kong Island protest last September.

The magistrate found the testimony by a male police officer to be unreliable due to a discrepancy with video footage and said the social worker had cooperated with the police at the time.

Hui, 50, was originally charged with assaulting a police officer. But before her trial started last Thursday, the prosecution changed it to a charge of obstructing constable Chong Tik-long in the execution of his duty last September in Queensway's lane near Rodney Street in Admiralty.

West Kowloon magistrate May Chung Ming-sun ruled that Chong's testimony was unreliable in yesterday's verdict. She said the time of interaction between Hui and Chong was about 10 seconds, which was insufficient time for the many actions that Chong claimed Hui had done - such as extending her hands to obstruct him and pushing him, making the officer fall twice.

After watching slow-motion footage, the magistrate deemed his falls to be accidental.

She said the video showed that Hui was surrounded by a group of police and Chong fell with her when they stepped on a concave part of a barrier on the road and lost balance.

The second time that Chong fell was because Hui's hands were raised in the air and another police officer slapped her left arm. Her arm then swung and hit Chong, causing him to fall.

The magistrate said the defendant had no intention of assaulting Chong. She added that Hui had been cooperative with the police and had followed their instructions to move.

The prosecution was not able to convince the court the accused person was guilty of the offense beyond a reasonable doubt, so the magistrate ruled that Hui was cleared of the charge.

The defendant sought legal costs, but the prosecution opposed it because Hui raised suspicions by walking toward a spot where the police kept those arrested on the street.

The magistrate ordered the prosecution to bear the legal costs as there was no evidence to support its claim since Chong's testimony was unreliable.

After leaving the court, Hui thanked her family and friends for their support. She felt she had suffered mentally and physically since the day of arrest.

"When I heard the result, tears fell," she said outside court. "Maybe those are happy tears. I'm not sure. All along, I knew that if I didn't commit any crime, I will have justice."

Hui Lai-ming

Social worker cleared of obstructing cops