A high-profile Hong Kong pro-independence leader said Tuesday he had been barred from standing in upcoming parliamentary elections -- the latest candidate backing separation from mainland China to be disqualified.

The apparent ban for Edward Leung, of the Hong Kong Indigenous party, from the September vote came despite him signing a controversial new form declaring Hong Kong is an "inalienable" part of China.

Critics have slammed the new stipulation by electoral authorities as political censorship and an attempt to deter prospective candidates from advocating self-determination or independence from Beijing.

Some activists are calling for more distance or even a complete breakaway from the mainland as fears grow that freedoms in the semi-autonomous city are disappearing due to Beijing interference.

Campaigners, including Leung, have challenged the declaration form in court and at least 13 prospective candidates have refused to sign it.

Leung, 25, eventually signed last week, despite his open advocacy for an independent Hong Kong, in the hope the authorities would validate his candidacy.

But he said he had been barred from standing.

"This election is a dark election, an election that is being controlled," Leung told reporters late Tuesday.

"Every day Communist China rules (Hong Kong)... I won't be able to enter the Legislative Council. So what else can I do? Revolution!"

His party accused the electoral commission of "trampling the will of the people, abusing administrative power and giving up political neutrality".

"There is no way the crime of selecting candidates according to political goals can be easily forgiven," it said in a statement.

The statement added a government officer handling Leung's case had explained she did not believe he had changed his pro-independence stance when notifying him by email that his application had been unsuccessful.

Leung later walked into a government briefing for election candidates and raised his middle finger at the speakers before departing to cheers and chanting from hundreds of supporters outside, some of whom wore balaclavas.

Other pro-democracy politicians also walked out of the meeting carrying banners saying "Defend fair elections".

The founder of the pro-independence Hong Kong National Party, Andy Chan, was one of three other hopefuls barred in recent days from standing in the September vote.

Chan had refused to sign the declaration form.

The other two prospective candidates disqualified were also part of the "localist" movement, which is pushing for more autonomy for Hong Kong after mass pro-democracy rallies in 2014 failed to win political reform.

Beijing and Hong Kong officials have repeatedly said advocating independence goes against the city's mini constitution, known as the Basic Law, and that independence activists could face legal consequences.

Various government departments including the electoral office made no comment Tuesday.

Hong Kong was returned from Britain to China in 1997 under an arrangement that guarantees civil liberties unseen on the mainland.

But concerns have grown that such freedoms are now fading as Beijing increases its influence across a range of areas, from politics to the media.