HONG KONG (REUTERS/AFP) - Six people were charged on Tuesday with conspiracy to cause an explosion, Hong Kong police said, amid high tension in the city ahead of a debate on a China-backed electoral reform package.

Police had on Monday arrested ten people on suspicion of conspiracy to manufacture explosives. 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.

In addition to the arrests police had also 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 pro-democracy rallies last year and are a symbol of anti-establishment defiance worldwide.

Police also said they would patrol inside the city's Legislative Council overnight ahead of Wednesday's debate, which sets out how Hong Kong will choose its next leader in 2017.

A vote is due by the end of the week. Democrats in the city's legislature have said they will vote against the plan, denying it the two-thirds majority it needs to pass.

Pro-democracy protesters are also staging evening rallies throughout the week.