The five democrats from the Civic Party and League of Social Democrats will resign from the 60-seat legislature on January 27 to force by-elections.
A spokesman for the legislators, Kenneth Chan, described their move as a de facto referendum on democracy.
It comes after a fresh political blueprint to tweak electoral arrangements in 2012 to move the city toward universal suffrage in 2017 disappointed many opposition democrats.
The resignation of the lawmakers and subsequent by-elections were a very important opportunity for Hong Kong citizens as a whole to express their strong desire for democracy for Hong Kong as soon as possible, Chan told reporters.
Hong Kong, a former British colony that returned to Chinese rule in 1997, has grappled with Beijing's Communist Party for more than a decade over a roadmap toward universal suffrage, as guaranteed in the city's mini-constitution.
The city's various opposition groups, known collectively as the pan-democratic camp and which controls 23 legislature seats, risks losing its crucial one-third veto bloc if the plan backfires and they fail to be re-elected.
We have nothing to lose. No pain, no gain, said To Kwan Hang, another spokesman for the resigning legislators.
The organizers called on Hong Kong people to unite and vote for the democratic lawmakers, who say they will declare the by-elections invalid if less than 50 percent of eligible voters cast ballots.
A convincing result in favor of pro-government and pro-Beijing lawmakers could see the remaining democrats in the legislature back down and support the contested blueprint for elections in 2012.
(Reporting by James Pomfret; Editing by Paul Tait)