Guinea-Bissau presidential candidate Domingos Simoes Pereira urged the nation to await official results after his opponent Umaro Cissoko Embalo said he was on course for victory.
Both candidates are former prime ministers.
“There is a lot of fake news circulating, but only official numbers can be respected,” Pereira said on Twitter on Monday.
Official results are expected on January 1.
Earlier on Monday, Embalo’s campaign spokesman Djibril Balde said they were ahead in Sunday’s runoff election.
Djibril Balde said the announcement was based on preliminary results observers had compiled in each region, and he claimed that Pereira had earlier called Embalo to congratulate him on his win.
Embalo, 47, came second in last month’s first-round vote with 28 percent versus 40 percent for Pereira.
Before Sunday’s runoff, he won the backing of incumbent President Jose Mario Vaz, who was eliminated in the first round with a vote of 12 percent.
Vaz’s five-year term was marred by regular political sackings, an ill-functioning parliament and corruption.
Voters said they hoped the next president would restore calm to the West African nation.
Embalo and Pereira both served as prime ministers under Vaz.
Guinea-Bissau has a long history of military coups and political assassinations since winning independence from Portugal in 1974.
Vaz was the first president in 25 years to finish his term without being overthrown or killed.