Liberians are voting in 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 the end more than 20 years ago of Liberia’s civil wars in which 250,000 people died.
The BBC’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”.
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 Weah has been talking about improving education and dealing with unemployment.
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.