The violence erupted after officials tried to clear illegal street hawkers from a busy commercial neighbourhood during Chinese new year, with police firing warning shots into the air.
Beijing officials blamed the clashes, which saw some 30 people arrested and charged with rioting, on "radical separatists".
Some participants were from "localist" groups pushing for more freedom for the semi-autonomous city or even a split from Beijing.
A magistrates' court sentenced Chan Pak-yeung, 31, an active member and online radio presenter for the localist political party Civic Passion, to nine months in prison for resisting and assaulting police, the party said.
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But Civic Passion questioned the evidence against Chan after the ruling.
"This must be political prosecution," said the party's vice chairman Alvin Cheng.
Cheng said he was worried that there would be more jail sentences over the riots.
Tensions are high in the city, which is ruled under a "one country, two systems" deal set up when it was handed back to China by Britain in 1997.
That agreement guarantees Hong Kong's freedoms and way of life for 50 years, but there are growing concerns that Beijing is increasing its influence.
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