In a rare weekend lull in the unrest that has roiled the financial hub, democratic candidates across the city of 7.4 million secured more than half of the 452 district council seats for the first time, against a strongly resourced and mobilised pro-establishment opposition.
When the results began trickling in after midnight, including upset wins for democrats against heavyweight pro-Beijing opponents, some voting centers erupted in loud cheers and chants of "Liberate Hong Kong. Revolution Now," -- a slogan of many protesters on the streets over the past half year.
Some winning candidates said the result was akin to a vote of support for the demonstrators, and could raise the heat on Hong Kong's pro-Beijing chief executive, Carrie Lam, amid the city's worst political crisis in decades.
"This is the power of democracy. This is a democratic tsunami," said Tommy Cheung, a former student protest leader who won a seat in the Yuen Long district close to China's border.
The voting ended with no major disruptions in a day that saw massive, though orderly queues form outside voting centers.
As of 6:00 a.m. (2200 GMT), pro-democracy candidates had secured a clear majority with more than 300 of 452 seats, compared to 41 seats for the pro-establishment camp, according to local media estimates. Four years ago at the previous such polls, democrats only secured around 100 seats.
Almost three million people voted, a record turnout of more than 71% that appeared to have been spurred by the turmoil, almost double the number last time.
Hong Kong's district councils control some spending and decide a range of local livelihood issues such as transport. They also serve as an important grassroots platform to radiate political influence in the China-ruled city.
Demonstrators are angry at what they see as Chinese meddling in the freedoms promised to the former British colony when it returned to Chinese rule in 1997. They say they are also responding to perceived police brutality.
China denies interfering and says it is committed to the "one country, two systems" formula for the autonomy of Hong Kong. Police say they have shown restraint in the face of potentially deadly attacks.
Jimmy Sham, a leader of the Civil Human Rights Front, which organised some of the anti-government rallies, won his electoral contest, as did Lester Shum, a former student leader of the umbrella movement in 2014, and Kelvin Lam, who stood in after activist Joshua Wong was barred from running.
A number of pro-Beijing heavyweights including Junius Ho, whose abrasive public comments have made him a hate-figure amongst many protesters, lost to pro-democracy challengers. He described it as "an exceptional election, and an unusual result" on Facebook.The protests started over a now-withdrawn extradition bill that would have allowed people to be sent to mainland China for trial but rapidly evolved into calls for full democracy, posing the biggest populist challenge to Chinese President Xi Jinping since he came to power in 2012.