HONG KONG (AFP) - Four suspects charged over an alleged bomb plot in Hong Kong were denied bail on Wednesday in a case which has been met with scepticism by pro-democracy campaigners.

Police made the arrests last month days before lawmakers voted on a controversial Beijing-backed political reform package which had sparked mass street rallies at the end of last year.

Five men and one woman, aged 21 to 34, have been charged with conspiracy to cause an explosion after police said they found explosive chemicals during a raid.

Four of the five men applied for bail but were denied. The fifth man also remains in custody. The woman had already been released on bail.

The five men have been held in custody for more than a month since their arrests mid-June.

"Why did you appear at a place where explosives were being tested? Why did you enter the location?" questioned magistrate Don So as he denied the bail applications.

"Only you were found at the scene," he added.

Police have said they seized chemicals which could be used to make explosives in the June raid and that one of those arrested admitted to being part of a "radical local group".

Local media named the group as the National Independent Party and linked it to a new "localist" pro-democracy movement which is seeking a more independent Hong Kong.

But activists accused the authorities of a smear campaign, with many saying they had never heard of the organisation.

The chemicals were seized at an abandoned television studio in the eastern district of Sai Kung, with some detonated at the scene.

A house search led to the seizure of ingredients which police said could be used to make TATP, a highly-volatile explosive believed to have been used in the deadly terror attacks in London in 2005.

Air rifles and "V for Vendetta" style face masks were also found. The masks were occasionally worn by protesters during the mass rallies and are a symbol of anti-establishment defiance worldwide.

During the suspects' first court appearance on June 17, they accused police of assaulting them or forcing them to cooperate by threatening them, the South China Morning Post reported at the time.

The suspects gave no further evidence on Wednesday.

The case was adjourned until September 4.

Investigators are still testing the chemicals seized, prosecutor Noelle Chit said.

They are also examining websites which the suspects browsed and messages they sent, as well as trying to access their phones and computers, she added.

Opposition legislators voted down the government's controversial reform bill in June on how Hong Kong would choose its next leader in 2017.

They labelled it "fake democracy" as it adhered to a ruling from Beijing that candidates would be vetted by a loyalist committee.

It was that ruling which sparked last year's mass protests which brought parts of the city to a standstill.