Police have arrested a tenth person suspected of making explosives in Hong Kong in a plot they say is linked to a "radical" political group, ahead of a key vote on a controversial reform package.  

A 58-year-old man was arrested "on suspicion of conspiracy to manufacture explosives" at northern border point Lo Wu late Monday, police said.   

They had already arrested nine people on Monday on the same charge. All the suspects were detained overnight.

Police said Monday that one of those arrested had claimed to be a member of a "radical local group".

They would not name the group or specify motives but said that maps of central districts of Hong Kong had been found and warned anyone taking part in public gatherings to stay away from "violent protesters".

It comes as a series of rallies take place before a vote expected Friday in Hong Kong's legislature on a divisive roadmap for the city's electoral system, which led to mass protests at the end of last year.

Chemicals were seized at an abandoned television studio in the eastern district of Sai Kung as part of the operation Monday, with some detonated at the scene.

A house search later led to the seizure of ingredients which police said could be used to make the powerful explosive TATP.

Maps of central neighbourhoods were also found, as well as a number of air rifles and face masks.

Local media named the radical 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 pro-democracy and localist campaigners questioned the link.

"People have never heard of the organisation and don't know its members.

"The whole thing could be a set-up intended as a smear campaign against the localist camp," said commentator Kam Sai-kit, writing for pro-democracy portal Post852.

The reform bill to be voted on lays out a proposal for choosing the city's next leader by public vote for the first time in 2017.

But it sticks to a ruling from Beijing which stipulates that candidates must be vetted by a loyalist committee.

That ruling sparked mass rallies and roadblocks towards the end of last year, with campaigners dismissing it as "fake democracy".  

Pro-democracy legislators are vowing to block the proposal.