Postings by pro-democracy users on the popular board LiHKG urged voters to go to polling stations early and for supporters "not to jeopardise the election".
Polls open Sunday morning as the semi-autonomous city of 7.5 million.
The district council polls have in the past generated little passion, dominated by candidates allied to the China-backed government with a remit over more mundane matters as rubbish collections and planning decisions.
But with protests coursing through the city, pro-democracy candidates are hoping to make a statement to Chief Executive Carrie Lam and the pro-Beijing government, which have refused to concede to democracy demands.
The poll to choose 452 councillors across 18 districts is the closest voters in Hong Kong get to direct representation -- but turnout is usually unremarkable.
Nearly 400,000 new voters have registered, however, which is widely interpreted as a positive sign for the pro-democracy camp.
An unprecedented police presence will be deployed at polling stations and on the city streets on Sunday to prevent any drama.
Lam's Chief Secretary Matthew Cheung called on citizens to let the polls proceed smoothly.
"If you create massive chaos or carry out unlawful deeds it will be difficult to hold a fair election," Cheung said.
"It's a real democratic exercise. I really want people to treasure it." Hong Kong's political unrest billowed out in June in opposition to an extradition bill to China.
Although the bill was eventually withdrawn, the movement snowballed into wider calls for democracy and investigations into alleged police brutality.
Protester Chow Pak-kwan, a 21-year-old who lost a kidney and part of his liver after being shot by a traffic police officer earlier this month, on Saturday echoed the calls for "peaceful" elections.
"I hope Hong Kong people can cast their votes to earn more democracy in a peaceful way," Chow Pak-kwan, masked and dressed in the black colours of the city's protest movement, told reporters.
"Every vote matters."
In a sign of mistrust between much of the public and the authorities, polling authorities on Saturday issued a statement to knock down rumours that facial recognition technology would be installed at polling stations.
The district polls remain the city's freest. In contrast, members of the city's legislature are elected by a mix of popular vote and industry groups stacked with Beijing loyalists.
The city's chief executive is chosen by a similarly pro-establishment committee.
Political analysts expect pro-democracy candidates to make gains in the district councils, but to still fall well short of a majority of the 452 slots.
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