Over a thousand people converged on Hong Kong's Legislative Council early on Wednesday before lawmakers debate a Beijing-backed electoral reform plan that could trigger fresh protests in the Chinese-controlled city.
The former British colony has reinforced security after mass protests crippled parts of the Asian financial hub late last year, presenting China's ruling Communist Party with one of its biggest political challenges in decades.
The Legislative Council issued an "amber alert" before the vote on the reform package, which is expected by Friday. The council was expected to begin debating the plan on Wednesday afternoon.
Police were deployed inside the council complex overnight, and police sources said more than 5,000 specially trained officers would be on standby, while some roads leading to government buildings were closed.
Activist groups said they expected 100,000 protesters to show up on Wednesday, although Beijing supporters easily outnumbered opponents of the package early 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 Chan, who was waving a Chinese flag.
Legislators will debate a blueprint that will allow a direct vote for Hong Kong's next leader in 2017, but only from pre-screened, pro-Beijing candidates. Opponents say they want a genuinely democratic vote.
Scores of pro-Beijing activists shouted and booed from behind a barrier as pro-democracy lawmakers entered the building. Others, some elderly, squatted on the ground as temperatures hovered around 30 Celsius (86 Fahrenheit).
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".
Hundreds of police stood guard as more than a thousand pro-Beijing supporters gathered, some waving banners that said: "Support the reform". They also played the Chinese national anthem over loudspeakers close to government headquarters.
Tension has been running high, especially after 10 people were arrested this week on suspicion of explosives offences. Six of them have been charged with conspiracy to cause an explosion.
China's Foreign Ministry said there were "certain people who want to use a series of damaging acts" to disturb the debate but still hoped it could proceed smoothly.
The ruling Communist Party's official People's Daily said in a front-page commentary that the vote was fundamentally about ensuring long-term prosperity and stability. The proposed package, it added, was designed to be a broadly representative one all sides could accept while at the same time "reducing all sorts of risks universal suffrage could bring.
"Looking around the world, some countries' and regions' universal suffrage systems are not in line with the actual situation on the ground, causing social chaos, economic hardship and difficulties too numerous to mention," the paper said.
Beijing has tried to lobby Hong Kong's 27 pro-democracy lawmakers to back the blueprint.
Those democrats, who hold a crucial one-third veto bloc in the 70-seat Legislative Council, have so far pledged to oppose what they call a "fake" democratic model.
Hong Kong's Independent Commission Against Corruption has also said it was investigating allegations by an unidentified legislator that he was offered a bribe to vote for the package.
Hong Kong returned to China in 1997 under a "one country, two systems" formula that gives it a separate legal system and greater freedoms than the party-ruled mainland - and the promise of universal suffrage.
Thousands of activists blockaded major roads across Hong Kong for 79 days last year, defying tear gas and pepper spray, to press China to honour that promise.
While flawed, the package is still the most progressive electoral model ever offered by China's leaders in what might be a pilot for other cities within mainland China, according to a source close to Beijing's leadership.
If the plan is vetoed, Hong Kong's next leader will be selected as before by a 1,200-member committee stacked with pro-Beijing loyalists. Beijing would be unlikely to offer any fresh concessions to Hong Kong anytime soon.