Guyana’s electoral authorities set the stage for the re-election of the country’s president late Friday, when they declared the country’s ruling party the winner of the capital region, in a widely denounced process that threatens to isolate the small South American nation and new oil producer.

“A president sworn in on the basis of those results will not be considered legitimate,” the American, Canadian, British and European Union embassies said Friday in a joint statement.

Guyana in January began to export the first trickle of the massive crude reserves discovered off its coast, a move that is expected to transform the poor former British sugar colony into a petrostate in the coming years. A new term would give the president, David E. Granger, power to manage proceeds from those exports.

However, the flawed vote signals for many Guyanese a return to the political malaise of the 1980s, when the predecessor of Mr. Granger’s party ruled the country through a series of sham elections and with the support of the army.