Liberians are voting in a presidential run-off election after the two leading candidates were separated by just over 7,000 votes in the first round a month ago.
They are choosing between incumbent and one-time football star George Weah and former Vice-President Joseph Boakai.
The president narrowly won the first round but failed to get more than 50% of the vote, triggering the run-off.
Voting in October was fraught with allegations of fraud and violence.
The election commission said that nine of its temporary staff had been arrested over alleged ballot-tampering.
The UN reported clashes between supporters of rival opposition parties.
This is the fourth presidential election since Liberia's second civil war, which ended more than 20 years ago and resulted in the deaths of more than 50,000 people.
The OceanNewsUK's Moses Garzeawu in the capital, Monrovia, says the turnout for the run-off election is expected be high as Liberians are "hungry to vote".
Images of long queues of people at polling stations have been captured in and around the city.
The head of the national elections commission, Davidetta Browne-Lansanah, said: "We want to thank all Liberians who left their bed this morning. Stay on the line and vote."
Mr Weah, 57, who got 43.8% of the vote in the first round, and Mr Boakai, 78, who got 43.4%, have both been trying to build political alliances with the 18 other candidates who ran in the first round.
None of them received more than 3% of the vote.
Mr Boakai, who served as the vice-president to Ellen Johnson Sirleaf, Africa's first elected female head of state, has secured the endorsement of three of the four best performing candidates, according to the Reuters news agency.
In his campaigning, he has focused on investing in agriculture and infrastructure. Mr Boakai has also campaigned on the need to rescue the nation from what he calls "mismanagement" by Mr Weah's administration,
The president has been talking about improving education and dealing with unemployment.
He has asked voters for more time to see the results of his first-term promises, to root out corruption and improve livelihoods.
Liberia is still recovering from the impact of two civil wars between 1989 and 2003, and the Ebola epidemic that killed thousands of people between 2013 and 2016.
According to the World Bank, the West African nation's economy expanded by 4.8% in 2022, due to "mining and a relatively good agricultural harvest".
This is the second time the two men have faced each other in a presidential election run-off vote.
In 2017, Mr Weah triumphed over Mr Boakai, gaining 61% of the vote in the second round.
That time, his international stardom helped his popularity among the youth and voters were also attracted by his promises to clamp down on corruption, analysts say.
Polls opened at 08:00 local time (08:00 GMT) and will close at 18:00 local time (18:00 GMT), when vote counting will get under way.
The victor will be sworn into office in January next year.
