Edward Leung, 24, one of the leaders of "localist" group Hong Kong Indigenous, is vying to become a legislator after a prominent pro-democracy politician stepped down.
Student Leung is taking on six other contenders for the New Territories East seat in the north of Hong Kong, including candidates from pro-Beijing and pro-democracy camps.
Since massive rallies in 2014 calling for fully free leadership elections failed to win concessions from Beijing, young protesters like Leung have become increasingly disillusioned with the more established pro-democracy parties.
Their frustrations spilt over into violence earlier this month when running street battles left more than 100 injured after masked protesters hurled bricks at police, who fired warning shots in the air.
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Leung is facing a rioting charge for his involvement.
"Hongkongers are the masters of their own soil," said Leung Sunday.
"I will defend Hong Kong people's interests and Hong Kong people's rights," he told reporters.
Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 and its freedoms are protected by a 50-year agreement.
The disappearance of five Hong Kong booksellers known for publishing titles critical of Beijing has exacerbated fears.
Four of the men are now under official investigation on the mainland and the fifth has said he is "assisting" authorities.
Leung is not tipped to win but his candidacy will be a barometer of how much support the fledgling localist movement can garner at the ballot box.
"I voted for him. We need to speak out about what's happening," said a 56-year-old engineer surnamed Cheung.
Others urged calm. "We should not be radical," said one 40-year-old voter surnamed Yeung.