About 3.4 million people are expected to vote in the presidential and legislative ballot after a campaign marred by tensions that led to the cancellation of rallies in the final stages and clashes at an opposition gathering on Wednesday.
Twelve men and one woman are vying for the top job and incumbent Bio's main challenger is Samura Kamara of the All People's Congress party, who narrowly lost to Bio in the last election in 2018.
While no leader in office has lost the presidency in the West African nation's recent history, the APC has banked on an enduring cost-of-living crisis tipping the balance in its favor. Inflation soared to its highest level in over 20 years in 2022, while the national Leone currency slumped 60% in value.
Hundreds waited in line at a polling station in a hilltop district of the capital Freetown, including first-time voter Abu Koroma, 23, who arrived in darkness two hours before polls opened.
"We have been yearning for change," he said, still in line by mid-morning. "We've had different leaders since gaining independence, but we are yet to have sustainable electricity, sustainable health, all these things a young person like me needs."
27-year-old Boubacar Conteh, from Wellington in the east of Freetown, waited since four in the morning to cast his ballot.
"I want my country to change - I need change," he said.
The current downturn has stalled hopes of economic progress in the wake of the devastating Ebola epidemic that peaked in 2014 and the 1991-2002 civil war. Widespread underemployment persists with over half of the population living in poverty, according to the World Bank.
Rising food prices are a key issue for many voters in the import-dependent nation of eight million people. Year-on-year inflation hit 43% in April, according to the latest official figures.
Both Bio and Kamara told AFP they would prioritize boosting agricultural production.
Two days ahead of the vote, Information Minister Mohamed Rahman Swaray said if Bio, 59, retains the presidency, he will focus on job creation and agricultural development to kickstart growth and improve living standards.
"We will do a lot more things to ensure people are comfortable," he told Reuters in an interview in a Freetown cafe.
Unusually violent protests last year over rising prices have raised fears of political unrest. Bio and Kamara reported small-scale attacks on their supporters ahead of election day, while the APC's recent questioning of the independence of election officials has raised tensions.
Both sides have called for calm. But there are concerns the situation could deteriorate, particularly if no candidate secures 55% of votes cast, triggering a runoff between the top two candidates two weeks after the announcement of the first-round results.
Information for this report came from Reuters and Agence France-Presse.