Pro-democracy activist Koo Sze-yiu went behind bars in the lead-up to the Lunar New Year after being jailed for nine months for desecrating the national and SAR flags.
The married 66-year-old construction worker was convicted on four counts - three of desecrating the national flag and one of doing the same to the Special Administrative Region colors - during protests on June 10 last year and on January 1.
He had pleaded not guilty to the charges.
Eastern Court magistrate David Chum Yau-fong said it was necessary to impose a heavy sentence as during one of the protests Koo set fire to a flag made of plastic, which was a threat to public safety.
Lawmaker "Long Hair" Leung Kwok-hung, supporting Koo, said the sentence was too heavy.
Koo set fire to a national flag during a rally on June 10 outside the central government's liaison office in Western District.
About 20,000 protesters were demanding that mainland authorities probe the death of dissident Li Wangyang in Hunan. It was ruled to be suicide.
On January 1, Koo was seen displaying damaged national and SAR flags in a march calling on Chief Executive Leung Chun-ying to step down.
Chum said Koo had acted recklessly in setting fire to the national flag because he had been ignoring the safety of others.
On hearing the sentence, Koo shouted slogans calling for an end to Communist Party rule and about "shameful" political suppression.
Before the hearing, Koo had joined in what was described as a celebration with other activists, including Leung, outside the court.
Later, Leung remarked: "The sentence is too heavy. The magistrate simply doesn't understand that in burning the national flag Koo was venting his anger at the suspicious death of Li."
The League of Social Democrats legislator, who plans to visit Koo behind bars today to check if he wants to mount an appeal, also said the activist did not mind being in jail over the Lunar New Year.
Koo was among 14 activists whose voyage to the disputed Diaoyu Islands in August sparked tension between China and Japan.
Lawyer Wong Kwok-tung, who was not involved in the case, said the heavy sentence appeared to be a warning to activists.
The maximum penalty for desecrating the national or the SAR flag is three years in jail and a fine of HK$50,000.