A Hong Kong activist who seeks independence from China and has been charged over recent street battles with police stood for office Sunday in a by-election that highlights the city's political faultlines.

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, including candidates from pro-Beijing and pro-democracy camps.

He is not tipped to win but his candidacy will be a barometer of how much support the fledgling localist movement -- fiercely opposed by Beijing -- can garner at the ballot box.

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 more established pro-democracy parties.

Smaller groups have emerged under a "localist" banner, pushing for greater autonomy and even independence from China as fears over interference from Beijing grow.

Their frustrations spilt over into violence earlier this month when running battles with police left more than 100 injured.

Leung is facing a rioting charge for his involvement.

"Hongkongers are the masters of their own soil," he said Sunday. "I will defend Hong Kong people's interests and Hong Kong people's rights."

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.

But there is growing concern those freedoms are under threat as China seeks to stamp its authority on the territory.

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.

"I voted for him (Leung). We need to speak out about what's happening," said a 56-year-old engineer surnamed Cheung.

"It's completely understandable that young people want change," Cheung told AFP.

Others urged calm. "We should not be radical," said one 40-year-old voter surnamed Yeung. "Any problems should be dealt with peacefully and rationally."

Observers say either Alvin Yeung of the established pro-democracy Civic Party or Holden Chow of the pro-Beijing Democratic Alliance for the Betterment of Hong Kong (DAB) are most likely to win.

Yeung criticised Leung's involvement in the street battles and defended the pro-democracy movement's peaceful track record.

"I understand people are not satisfied, but... we need principles and our bottom line is non-violence," Yeung told AFP.

The pro-Beijing camp casts democracy campaigners as a threat to stability and prosperity in the city.

Beijing has dismissed localists as "separatists" -- a label they do not shy away from.

By mid-afternoon, 200,000 people had voted, with polls due to close late Sunday and the result expected overnight.

If Chow wins, the pro-democracy camp will still hold enough seats in parliament to block important bills, but will risk its ability to block other parliamentary decisions.