Ousted lawmaker Au Nok-hin was on Monday found guilty of assaulting two police officers with a loudhailer.
One officer was said to have been left with hearing loss because the megaphone was so loud, while Au struck the other officer's shield with the device.
The incidents happened during a standoff between protesters and police in the early hours of July 8 last year, following an anti-government rally in Yau Ma Tei.
Au had pleaded not guilty at Kowloon City Court.
In delivering the verdict, magistrate Leung Ka-kie said the officer whose hearing was damaged had repeatedly warned Au not to use the loudhailer so close to him.
Leung said the other officer had feared a violent attack because of the facial expressions and force Au had used when striking his shield with the loudhailer three times.
In mitigation, Au's lawyer said the former lawmaker is remorseful and he didn’t deliberately intend to injure anyone. The lawyer urged the magistrate to impose a non-custodial sentence.
Sentencing was adjourned until April 24.
Speaking outside court, Au said he would urge the court to consider the social situation that Hong Kong has been facing.
"At that time, we all know about the circumstances and what I had done," he said.
Au and fellow pan-dem Gary Fan lost their seats in Legco last December when the city's top court refused to hear an appeal against High Court rulings that their 2018 elections were invalid.
The High Court had found that Demosisto leader Agnes Chow and localist Ventus Lau were wrongly banned from running in the two by-elections, meaning Au and Fan were not duly elected.