HONG KONG (AFP) - Hundreds of protesters from both sides gathered outside the Hong Kong legislature on Wednesday morning, as the city prepared for a political showdown on Wednesday with lawmakers set to vote on a divisive reform package and tensions high over an alleged explosives plot that police said was linked to a "radical" group.

The warring protesters were kept apart by metal barriers while police guarded the entrance to the building. "I'm here to oppose the so-called democracy," said protester Fion Wong, in her forties. "Passing it (the Bill) woul be a betrayal to those who have taken part in the Occupy movement," she added, referring last year's mass protests.

Civic Party lawmaker Alan Leong urged cheering pro-democracy supporters to "continue to fight".

Meanwhile, pro-government supporters waved Chinese national flags and shouted "2017, make it happen!" through loudspeakers.

"This morning I got a very long and well-written letter, which was from a student. (They) hope I will support the proposal," said democrat Ronny Tong, who was close to tears. "I cannot think of anyone in Hong Kong who is happy today. I have been a legislator for 11 years, with an aim to fight for universal suffrage. Today I will cast a negative vote for an incomplete and unsatisfactory political reform proposal."

Activist groups said they expected 100,000 protesters to show up on Wednesday, although Beijing supporters easily outnumbered opponents of the package on Wednesday.

"The Bill needs to go through. We have to support Hong Kong stability. We cannot keep carrying on like this," said a 60-year-old man surnamed Chan, who declined to give his first name.

"We have worked so hard all these years," said Mr Chan, who was waving a Chinese flag. Pro-Beijing supporters also played the Chinese national anthem through loudspeakers.

Legislators started debate on the blueprint on Wednesday afternoon. Opponents of the plan say they want a genuinely democratic vote for Hong Kong's next leader.

Pro-Beijing supporters easily outnumbered democracy activists and, with temperatures hovering around 30 deg C, crowd numbers had dwindled by early afternoon.

One pro-democracy protester wearing a black T-shirt which read "Reject Fake Suffrage", held a black-and-white banner that declared: "Overthrow the Communist dictatorship".

Inside the legislature, democrats stood in a row with small signs with crosses on them as they pledged to vote down the plan.

The vote is expected to take place by Friday. It pits democracy campaigners against the government in the culmination of a fraught chapter which saw tens of thousands of protesters take to the streets last year over the controversial electoral roadmap.

Although the government's reform proposal would for the first time give all residents the right to vote for the chief executive in 2017, it adheres to a Beijing ruling that candidates must be vetted by a loyalist committee.

The plan is derided as "fake democracy" by opposition lawmakers, who have vowed to vote against it and deny the two-thirds majority it needs to pass.

The authorities in Hong Kong have said repeatedly that they cannot diverge from Beijing's August ruling, which sparked weeks of mass rallies that brought parts of the city to a standstill.

Political analyst Sonny Lo called the vote "a critical juncture" and said the Bill was unlikely to pass due to the pan-democrats' pledge.

That paves the way for a further schism between Hong Kong and Beijing, he said.

"Beijing will lay the blame on those democrats who torpedo the political reform Bill, which (it) believes represents a sincere concession from the central government."

It comes after police arrested 10 people "on suspicion of conspiracy to manufacture explosives", and seized materials they said could be used to make the highly volatile explosive TATP.

Police said one suspect had claimed to be a member of a "radical local group" named by local media as the National Independent Party.

The group was reportedly linked to the pro-democracy "localist" movement that is seeking a more independent Hong Kong.

Five men and one woman were jointly charged with conspiracy to cause an explosion and were due to appear in court Wednesday, while one man and three women were freed on bail pending further investigations.

But activists accused the authorities of a smear campaign, with many saying they had never heard of the group.

The Hong Kong government has said political reform would be off the table if the current package is rejected.

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

Some fear more radicalism could emerge if the impasse continues.

"With the current political deadlock... Hong Kong is moving to a situation where it could be unstable and could be more violent," said Dr Surya Deva, an associate professor at Hong Kong's City University school of law.

Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 and sees much greater freedoms than on the mainland, but there are fears that those are being eroded.