Hong Kong student activist Tony Chung has been sentenced to four months in prison for insulting the Chinese flag during a protest in May last year.
The 19-year-old had faced a sentence of up to three years in jail after he was found guilty earlier this month.
The court heard Mr Chung had 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 protest that led to Tuesday's sentence was among the earliest against a controversial extradition bill put forward by the Hong Kong government, which was eventually withdrawn in the face of a mass demonstration movement.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, was convicted on charges of desecrating the flag and unlawful assembly. 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.
The law has drawn global criticism and accusations that it ends freedoms guaranteed for 50 years after British rule ended in Hong Kong in 1997. The Chinese government has dismissed the criticism, saying the law is necessary to prevent the kind of pro-democracy protests seen in Hong Kong throughout 2019.
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 was arrested again in October by plainclothes police officers near the US consulate in Hong Kong. The UK-based activist group Friends of Hong Kong said he had been planning to enter the consulate to claim asylum.
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.