The figures seemed to suggest that Barrow was poised for re-election as early results from most districts placed him in the lead after Saturday's crucial polls for the young democracy.
But his challengers rejected the results, main rival Ousainou Darboe and two other candidates saying in a joint statement: "All actions are on table."
"At this stage we reject the results announced so far" by the electoral commission, Darboe told reporters, his two fellow candidates by his side.
Some of Barrow's supporters however, were already beginning to celebrate victory in the streets of the capital Banjul.
Barrow, who ousted dictator Yahya Jammeh five years ago, was well ahead of Darboe in results published late Sunday afternoon by the electoral commission for 50 districts out of 53 nationwide.
The figures cover more than half of registered voters in the single-round ballot.
An electoral commission official told reporters in Banjul that final results should be announced later in the day.
The election is being closely watched as a test of the democratic transition in The Gambia, where Jammeh ruled for 22 years after seizing power in a bloodless coup in 1994.
Jammeh was forced into exile in Equatorial Guinea in January 2017 after Barrow, then a relative unknown, defeated him at the ballot box.
Barrow, 56, faced five challengers in his re-election bid.
Test of democracy
Several factors have slowed the tally, including The Gambia's scarce financial resources, high turnout and the country's unusual voting system.
Illiteracy is widespread in The Gambia, so voters cast their ballot by dropping a marble into a tub marked with their candidate's colour and photo -- a practice dating back to the country's past as a British colony.
Many of the roughly one million eligible voters in the nation of more than two million people are hoping for an improvement in their living standards.
The Gambia, a sliver of land about 480 kilometres (300 miles) long surrounded by Senegal, is one of the poorest countries in the world.
About half of the population live on less than $1.90 per day, the World Bank says.
The tourism-dependent economy was dealt a severe blow by the Covid-19 pandemic.
Barrow ran on a continuity ticket, pointing to infrastructure projects completed under his watch, as well as increased civil liberties.
Political veteran Darboe is a lawyer who has represented opponents of Jammeh, and who ran for president against the former dictator several times.
He also served as foreign minister and then vice president under Barrow before stepping down in 2019.
Ernest Bai Koroma, head of an election observation mission from the Economic Community of West African States (ECOWAS), "appealed to all the candidates to accept the outcome of the election in good faith" in a statement Sunday.
"There will be no winner or loser but only one winner, The Gambian people," he added.
Jammeh lost to Barrow in the 2016 election, but had to be removed by a military intervention from other west African states.
Barrow himself has already gone back on a promise to remain in power for only three years, and has weakened rhetoric about prosecutions for crimes committed under Jammeh.
Questions over Jammeh's continuing role in politics, and his possible return from exile, were central themes in the run-up to the election.
In September, Barrow's NPP party announced a pact with Jammeh's APRC -- a controversial move that was viewed as an electoral ploy.
Jammeh said the decision had been taken without his knowledge, and his supporters formed a rival party. But rights groups fear the pact will diminish chances of a trial.
The former dictator retains significant political support in The Gambia and has sought to influence the vote, remotely addressing rallies of supporters during the campaign period.
A truth commission Barrow set up to probe alleged abuses under Jammeh's rule heard testimony from hundreds of witnesses about state-sanctioned death squads, witch hunts and forcing bogus cures on AIDS patients.
The commission recommended in November the government pursue criminal charges, in a final report delivered to Barrow but not released to the public.