Guinea-Bissau's presidential candidate, Domingos Simoes Pereira, urged the nation to expect official results after his opponent Umaro Cissoko Embalo said he was on his way to victory.
Both candidates are former prime ministers.
"There is a lot of false 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, the Embalo campaign spokesperson Djibril Balde They said they were ahead of Sunday’s second round.
Djibril Balde said the announcement was based on preliminary results that observers had compiled in each region and said Pereira had previously called Embalo to congratulate him on his victory.
Embalo, 47, ranked second in the first round of last month's vote with 28 percent versus Pereira's 40 percent.
Before the second round on Sunday, he won the support of current President José Mario Vaz, who was eliminated in the first round with 12 percent.
Vaz's five-year term was marred by regular political layoffs, a poorly functioning parliament and corruption.
Voters said they expected the next president to restore calm in the West African nation.
Embalo and Pereira served as prime ministers under Vaz.
Guinea-Bissau has a long history of military coups and political assassinations since becoming independent from Portugal in 1974.
Vaz was the first president in 25 years to finish his term without being overthrown or killed.