HONG KONG (Reuters) - Dozens of police stood guard around Hong Kong government headquarters on Tuesday, a day after authorities arrested 10 people and seized suspected explosives ahead of a crucial vote on a China-backed electoral reform package this week.

The Chinese-ruled city is bracing for a fresh democracy showdown after protests crippled parts of the former British colony last year, and resulted in sometimes violent clashes with police, over plans for how its next leader will be elected.

Hong Kong's legislature is due to begin debate on the electoral reform package in the Legislative Council on Wednesday, with a vote due by the end of the week. Pro-democracy protesters are staging evening rallies throughout the week.

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

Police said the 10 people were arrested on Monday on suspicion of conspiracy to manufacture explosives and that some belonged to a radical group.

They did not elaborate and no charges have been laid.

Hong Kong media reported on Tuesday that some of those arrested belonged to a little-known group called the National Independent Party. 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".

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.

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.

"The timing of the arrests is obviously of interest: especially in this politically charged week before the LegCo vote," said Vickers, who is also former head of the Royal Hong Kong Police Criminal Intelligence Bureau.

Beijing has proposed a direct vote for Hong Kong's next leader in 2017, but only among pre-screened, pro-Beijing candidates. Democracy activists say they want a genuinely democratic vote.

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.