Hundreds of protesters marched through Hong Kong's main streets on Sunday against a ban on pro-independence candidates from running in an upcoming legislative election, as fears grow over Beijing's influence in the city.

In the last month five candidates who advocate a split from China were rejected from standing in the September 4 vote, with officials saying their stance went against Hong Kong's mini-constitution.

Critics have slammed the move as censorship as fears grow over Beijing interference in the semi-autonomous city in a range of areas, from politics to media and education.

The idea of independence is dismissed as illegal by Beijing and Hong Kong authorities, and was a taboo subject until recent months, when new parties emerged campaigning for a breakaway.

"I hope that people are aware of the political vetting. It is something that every Hongkonger should be concerned about because damage has been done to the rule of law as well as fairness in elections," protest organiser Jimmy Sham Tsz-kit, of Civil Human Rights Front, told reporters before the rally.

Marchers held banners with the slogans "No political vetting!" and "Watch out! Iranian-style election in Hong Kong!" and shouted: "Give me back fair elections!" and "Defend freedom of speech".

"Talking about independence is part of freedom of speech. I'm worried that in future people will have to watch what they say," 20-year-old student Arry Tsui told AFP.

Police said up to 760 people participated in the protest, with organisers claiming more than 1,000 supporters.

Support for the independence movement has surged since the five were banned from standing in the elections.

Earlier this month, they led the city's first pro-independence rally as tension over the upcoming vote escalates.

The growth of "localism", and new calls for self-determination and independence, grew out of frustration among many young activists at the failure of mass pro-democracy rallies in 2014 to win any political reforms from Beijing.

The city was returned from Britain to China in 1997 with an arrangement that guaranteed civil liberties.