Young Hong Kong independence activists calling for a complete break from China stood in major elections for the first time Sunday, the biggest vote since 2014 pro-democracy rallies.

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.

Most established pro-democracy politicians do not support the notion of independence, which until recently was taboo.

There is concern that the veteran democratic camp may lose seats to voters who now favour more radical new groups.

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.

Now some young campaigners are demanding outright independence, others the chance for Hong Kong to determine its own future in a referendum.

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.

One 30-year-old voter who gave her name as Sandy said she favoured independence.

"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.

"It is too idealistic and unrealistic," he told AFP.

- Pro-Beijing system -

Even if localists did win seats, with their numbers still small, they would not tip the balance in a system where it is almost impossible for the anti-Beijing camp ever to gain a majority.

While 40 of the Legislative Council's 70 members are directly elected by the public, 30 are selected by small voting blocs from special interest groups representing a range of businesses and social sectors. Those seats always go predominantly to pro-Beijing candidates.

Casting his vote Sunday morning, Hong Kong's unpopular leader Leung Chun-ying, who is seen by critics as a Beijing stooge, assured the elections were "democratic".

Several political opponents protested outside the polling station, with one throwing a tuna sandwich towards Leung, saying it symbolised the fact that elderly people cannot afford to eat breakfast in a city where the wealth gap is widening.

Entrenched divisions have led to a Legislative Council often hamstrung by filibustering and point-scoring.

With housing and low salaries serious concerns, many frustrated residents say it is time to put politics aside and focus on struggling communities.

"I just hope that people can sit down and talk without going radical," said a 72-year-old voter surnamed Yau, who added regular weekend protests have made him worried about where to take his grandchildren.

Yau said he had voted for a "peaceful" candidate.

Polls will close at 10:30 pm (0230 GMT). The vote count begins soon after and results are expected early Monday.