To Wing-cheung, 60, pleaded guilty to behaving in a disorderly manner in a public place, after chanting anti-police slogans in Mong Kok on June 12.
West Kowloon Magistracy heard that around 50 people were gathered by an exit of Mong Kok MTR Station when To started his chants, with some of the crowd joining in.
Police warned them to stop and To fled, but he was eventually arrested.
In mitigation, To’s lawyer said his client had acted on impulse and was affected by the socio-political atmosphere at the time.
He said To was remorseful, adding that the conviction means he could lose part or all of his pension.
But principal magistrate Ivy Chui said a deterrent sentence was necessary as the retiree had defied police orders.
She added that To’s action could have triggered the emotions of other protesters.