By Anne Marie Roantree and Clare Baldwin

HONG KONG (Reuters) - Hong Kong's leader warned on Tuesday that violence will not be tolerated as six people were charged with conspiracy to cause an explosion ahead of a crucial vote on a China-backed electoral reform package this week.

Security has been stepped up across the Chinese-ruled city, including at government buildings and train stations, as it braces for a fresh showdown over plans for how its next leader is elected in 2017.

Authorities are taking no chances after mass pro-democracy protests crippled parts of the former British colony late last year and presented China's Communist Party leadership with one of its biggest political challenges in decades.

"Hong Kong society should not tolerate any illegal activities. Whether these are violent or non-violent, we should not allow any illegal activities to be justified," Chief Executive Leung Chun-ying said in a televised briefing.

Ten people were arrested on suspicion of conspiracy to manufacture explosives, police said on Monday, adding that some belonged to a radical group. Six people were charged on Tuesday with conspiracy to cause an explosion.

Police said they would patrol inside the city's Legislative Council overnight ahead of the debate due to start on Wednesday. A vote is due by the end of the week. Pro-democracy protesters are staging evening rallies throughout the week.

Beijing has proposed a direct vote for Hong Kong's next leader in 2017, but only from among pre-screened, pro-Beijing candidates. Democracy activists say that makes a mockery of China's promise of universal suffrage.

"If the pan-democrats stubbornly insist on vetoing the proposal, democracy in Hong Kong will come to a standstill," said Song Ru'an, a Chinese Foreign Ministry official in Hong Kong.

The head of China's Hong Kong Macau Affairs Office, Wang Guangya, in an interview with two pro-Beijing newspapers, reiterated Beijing's desire to see the electoral package passed. He said that if the electoral framework were accepted, there would still be room for changes in future.

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With tensions running high before the debate, Hong Kong's Independent Commission Against Corruption has also said it is investigating allegations by an unidentified legislator that he was offered a bribe to vote for the package.

Some of those arrested in the raids belonged to a little-known group called the National Independent Party, media reported on Tuesday. According to its Facebook page the group was set up in January, but the page has now been deleted.

A June 1 post purportedly from the group warned that, if the reform package was passed, "Hong Kong people should be mentally prepared there will be casualties".

The Global Times, a widely-read tabloid published by the Chinese Communist Party's official People's Daily, said in an editorial that following the finding of the explosives, Hong Kong risked descending into chaos.

Rising tensions have resulted in a new front of radical activism in Hong Kong, where some groups have staged small but disruptive protests targeting mainland Chinese visitors.

Anger has also spilled over to soccer crowds, with supporters of the Hong Kong team loudly booing the Chinese national anthem on Tuesday night at the start of a local World Cup Asian zone qualifying match against the Maldives.

Some fans turned their backs and others chanted "we are Hong Kong" in a repeat of similar scenes last week before Hong Kong's match against Bhutan.

The fans' protests came despite warnings from a local soccer chief that such actions were disrespectful and might trigger an investigation from FIFA, the scandal-plagued world soccer authority.

Monday's raids by scores of officers rattled some legislators and residents. Posts on social media questioned the timing of the arrests, details of which were leaked to Hong Kong media before an official announcement.

Others were quick to sound a cautious note.

"I suggest we look very carefully and calmly at this case before we afford this incident too much priority or seek to amend the Hong Kong threat profile," said Steve Vickers, chief executive of risk consultancy SVA and former head of the Royal Hong Kong Police Criminal Intelligence Bureau.

Britain handed Hong Kong back to China in 1997 under a promise that core personal and commercial freedoms, backed by a British-style legal system, would be protected for 50 years.

Two senior police officers have told Reuters at least 5,000 officers will be standing by on the day of the vote. Authorities expect the scale of protests to be much larger, and possibly violent, if the reform package is passed.

(Additional reporting by James Pomfret, Yimou Li, Donny Kwok in HONG KONG and Ben Blanchard in BEIJING; Editing by Paul Tait and Nick Macfie)