Hong Kong student activist Tony Chung has been sentenced to four months in prison for insulting the Chinese flag during protests in May last year.
The court said the 19-year-old undermined the dignity of the national flag by pulling at it, breaking the flag pole, and throwing the flag in the air.
The teenager was already awaiting trial on a separate national security charge.
The sentence was linked to protests against the controversial extradition bill put forward by the Hong Kong government, which was eventually withdrawn in the face of a mass demonstration movement.
Mr Chung, the former leader of pro-independence group Studentlocalism.
China is increasingly cracking down on prominent pro-democracy activists in Hong Kong, causing some to flee the city.
While serving his sentence, Mr Chung will await trial on separate charges filed under a new national security law imposed by China in July, which criminalises subversion, secession and collusion with foreign forces.
Mr Chung was among the first to be arrested under the new law, in July, and he was the first to be prosecuted, on a charge of “inciting secession”.
He is also facing separate charges of money laundering and conspiring to publish seditious content.
In a separate case, 10 Hong Kong activists went on trial in mainland China on Monday. They were 10 of 12 activists arrested at sea as they tried to flee to Taiwan by speedboat – most already facing charges related to protests.