Hong Kong pro-democracy activists challenged controversial new election rules in court Wednesday after candidates for an upcoming vote were asked to sign a form saying the city is an "inalienable" part of China.

Critics have slammed the new stipulation as political censorship and an attempt to deter candidates in September's parliamentary elections from advocating self-determination or independence from Beijing.

It comes as some young campaigners 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.

At least 13 pro-democracy candidates have refused to sign the declaration.

Hong Kong's High Court said Wednesday it would not make a ruling on the challenge over the legality of the form -- brought by two pro-democracy political groups -- before the end of the nomination process Friday, as activists had wanted.

Instead the case was adjourned until August.

"The public should be angry...if candidates have to be screened based on their political views," activist Avery Ng of the League of Social Democrats told reporters, adding he was "disappointed" with the delay in the court decision.

Edward Leung of Hong Kong Indigenous -- a "localist" group pushing for independence from Beijing -- said everyone had the right to stand.

"This is definitely political censorship if someone is not approved to stand in the election," Leung told reporters outside court.

It is not yet clear whether those candidates who have refused to sign the form will be barred from running.

Some have told local media their candidacy has been confirmed despite opting out of the declaration. Others have said they have been quizzed by election officials over their stance on independence.

Leaders of several pro-independence groups have announced they are running for the legislature in September, as well as other pro-democracy campaigners who are calling for self-determination for Hong Kong.

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

Election authorities in Hong Kong introduced the new declaration form earlier this month. It sets out three constitutional points, including the description of Hong Kong as a "local administrative region" of China.

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.

That negative sentiment was exacerbated by the disappearance last year of five Hong Kong-based booksellers from a firm that published gossipy books about leading Chinese politicians.

All resurfaced on the mainland where they were investigated over trading banned books.