With about 85 percent of the votes from Sunday's election tabulated, the socialist Ortega had nearly 63 percent to 31 percent for his right-wing challenger, radio personality Fabio Gadea, The Daily Telegraph and CNN reported. Former President Arnoldo Aleman, whose conviction for corruption was overturned, was third with 6 percent.
CNN said electoral council President Roberto Rivas congratulated Ortega on gaining another term, his third counting an earlier stint as president from 1985-90.
But Gadea said the election results were tainted by fraud, the British newspaper and U.S. news network said.
"We can't accept the results presented by the Supreme Electoral Council because they don't reflect the people's wishes," he said.
International election observers said voting irregularities they detected were not serious enough to change the outcome, the Telegraph said.
Still, Eliseo Nunez of the Liberal Independent Party told CNN 20 percent of election observers were prevented from entering polling stations.
"This has been a process plagued with irregularities," he said.
U.S. State Department spokeswoman Victoria Nuland also expressed concerns about the allegations of voting irregularities and voter intimidation.
"Frankly, if the Nicaraguan government had nothing to hide, it should have allowed a broad complement of international monitors," she told reporters in Washington.