They are fighting for seats in the Legislative Council, or LegCo -- Hong Kong's lawmaking body -- as concerns grow that Beijing is tightening its grip on the semi-autonomous city.
Polling stations were busy as campaigners with megaphones urged residents to vote on a hot and humid day.
But wins for the young activists could split the democracy camp's vote -- and end up playing into the hands of pro- Beijing parties.
There is concern that the veteran democratic camp may lose seats to voters who now favour more radical new groups.
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If the democrats lose just four seats overall, they will forfeit the one-third voting bloc they need to veto bills, stacking the already skewed legislature even more in favour of Beijing.
Fears that Hong Kong's freedoms are disappearing were recently fanned after five city booksellers known for salacious titles about Beijing politicians disappeared, resurfacing in detention on the mainland, triggering widespread condemnation.
That fuelled the fire of the "localist" movement, which is seeking distance from China after the failure of the 2014 rallies to win concessions on political reform.
The more strident independence activists -- slammed by Beijing and Hong Kong authorities as acting illegally by promoting the breakaway -- were banned by the government from running in Sunday's election, a move which sparked outrage over political censorship.
Polls show some of the handful of pro-independence candidates running may win seats.
Hong Kong political analyst Joseph Cheng says he expects new faces in the legislature.
"This election is very much characterised by an inter-generational change of politicians and political leaders," he told AFP.
"This is a very critical time...We are here to ensure a voice can still be heard," she said.
But while victory for anti-China activists would be a massive coup, many still feel they are chasing an impossible cause.
Student voter Wilson Vai, 21, said he supported the pro-democracy camp -- but felt calling for independence was going too far.