Dozens of people joined queues at a polling station visited by AFP in the capital, Freetown, as early as 3 am, four hours before the vote kicked off.
First-time voter Joseph Kargbo, 18, said his mother had ordered him to vote for the opposition Sierra Leone People's Party (SLPP), which has promised free universal education - a key campaign issue.
"I just want to have change in our community and government," he told AFP.
"My mum can't pay the school fees, and the SLPP said when they come to power they will make primary and secondary education free," he added, showing off an index finger marked with ink-proof he had cast his ballot.
President Ernest Bai Koroma, who cannot run again after consecutive five-year terms, has anointed former foreign minister Samura Kamara as his successor for the ruling All Peoples Party (APC).
The export-dependent economy of the mineral-rich but impoverished country is in a dire state following the 2014-16 Ebola crisis and a commodity price slump that has driven away foreign investors.
Mohamed Conteh, chief commissioner of the National Election Commission, had said Tuesday that electoral authorities were "committed to conducting credible elections."
More than 3.1 million voters are registered for the polls. Partial tallies are expected within 48 hours and complete results within two weeks.
Kamara campaigned on continuity rather than change. "I am (Koroma's) biggest legacy," he told AFP at the final APC rally on Monday. "I need to sustain what he has achieved... and build on it."
The SLPP has retained the same candidate, Julius Maada Bio, who lost to Koroma in 2012. The National Grand Coalition, headed by former UN diplomat Kandeh Yumkella, is challenging the two-party system by appealing to young and better educated urban voters less likely to vote along regional and ethnic lines.