Hong Kong's government is set to present a controversial political reform package ahead of a much-anticipated vote later this week.
Protesters have gathered outside the city's Legislative Council building.
The package will for the first time give citizens the right to vote for its chief executive, in 2017. But candidates will be vetted by a pro-Beijing committee.
Last September the city saw weeks of large-scale pro-democracy protests.
Hundreds of activists, comprising of pro-democracy and pro-Beijing supporters, are outside the Legislative Council (LegCo) premises waving banners and shouting slogans.
"I'm here to oppose the so-called democracy," protester Fion Wong told AFP news agency. "Passing it [the bill] would be a betrayal to those who have taken part in the Occupy movement," she added, referring to the Occupy Central pro-democracy movement.
A pro-Beijing supporter surnamed Chan told Reuters: "The bill needs to go through. We have to support Hong Kong stability. We cannot keep carrying on like this."
The South China Morning Post reported that police have stationed at least 200 officers within the building and deployed another 1,000 outside.
Details of the package are expected to be presented by the government at 13:00 local time (05:00 GMT) on Wednesday.
Hong Kong's pro-democracy lawmakers look set to vote against the reforms despite warnings from the Chinese government not to do so.
If that is the case, it is unlikely to get the two-thirds majority it needs to pass.
"If the pan-democrats stubbornly insist on vetoing the proposal, democracy in Hong Kong will come to a standstill," Song Ru'an, a Chinese foreign ministry official in Hong Kong, told reporters.
It is unclear what China's response will be if the package is vetoed in the vote, which is expected to take place on Thursday or Friday.
Security has been stepped up across the city. On Tuesday, China warned against "radical forces" in Hong Kong after police arrested 10 people on suspicion of making explosives.