China warned Tuesday against "radical forces" in Hong Kong after police arrested 10 people on suspicion of making explosives ahead of a vote on a controversial political reform package.

Police said one suspect had claimed to be a member of a "radical local group" but would not name the group or specify motives.

They said late Tuesday that five men and one woman had been jointly charged with conspiracy to cause an explosion and would appear in court Wednesday, while one man and three women were freed on bail pending further investigations.

The bomb allegations have been met with scepticism by some commentators in the semi-autonomous Chinese city, who questioned the timing of the revelations before the vote expected by Friday.

But a senior Chinese official urged democrats to tread a "moderate" path.

"The radical opposition camp has recently resorted to despicable means... and are engaged in extreme violent activities," said Song Ruan, deputy commissioner for Beijing's foreign affairs office in Hong Kong, without specifically referring to the arrests.

"We hope that the moderate pan-democrats will see through the true face of radical forces."

Hong Kong's legislature tightened security, saying there was "a likelihood of the legislative council complex being stormed by radical groups" in the coming days.

Both pro-democracy and pro-government groups are due to rally outside the complex Wednesday, when debate starts.

Song urged lawmakers to pass the proposal, which sets out how Hong Kong will choose its next leader in 2017.

Democrats in the city's legislature have said they will vote against the plan, denying it the two-thirds majority it needs to pass.

Although the package will for the first time give all residents the right to vote for the chief executive, it adheres to a Beijing ruling that candidates must be vetted by a loyalist committee.

That ruling sparked mass protests and street blockades late last year by people decrying "fake democracy".

Currently the chief executive is chosen by a 1,200-strong pro-Beijing committee.

China has not made clear what its response will be should the package be vetoed.

"No-one knows when the constitutional reform process will be re-launched," said Song.

"The prospects for Hong Kong will be very worrying."

Daily rallies are being held before this week's vote.

- 'Smear campaign' -

In addition to the arrests police seized chemicals 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 that police said could be used to make TATP, a highly volatile explosive believed to have been used in the deadly jihadist 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.

One suspect, wearing a black hood, was taken back to the scene in Sai Kung Tuesday for a police reconstruction.

"I suggest we look very carefully, deeply and calmly at this case before we give this individual incident too much priority," security expert Steve Vickers said of the arrests.

"The timing is obviously of considerable interest."

Vickers said it was easy to find the ingredients for TATP online but it would be difficult to manufacture the explosive.

"The persons who are accused of possessing this, unless they are experts, probably present a greater threat to themselves than the public if they try and transport this material," said Vickers, a former senior police officer.

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

A Facebook page purportedly for the group, which only had posts from within the last 24 hours, showed an image of the Statue of Liberty carrying a sickle.

A previous page for the group, created in January, was taken down Monday.

Local media quoted the old page as saying: "Hongkongers must be prepared that there would be casualties" on June 17, the day the reform bill goes to the legislature for debate.

But there was scepticism from some campaigners.

"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 political commentator Kam Sai-kit, writing for pro-democracy portal Post852.