International observers on Friday called on Guyana President David Granger not to claim victory until election results can be verified, due to "credible" allegations of fraud.
Granger's ruling Partnership for National Unity and Alliance for Change had a clear lead in partial results published Thursday from the oil-rich South American country's largest region.
But international observers say giving out partial results is illegal and have implored the government to respect the law.
"We call on President Granger to avoid a transition of government which we believe would be unconstitutional as it would be based on a vote tabulation process that lacked credibility and transparency," representatives from the United States, Britain, Canada and European Union said in a statement.
They expressed deep concern over "credible" allegations of election fraud that could influence the results.
"We call on all to ensure proper procedures are in place to yield a credible election result. A fair and free process is vital for the maintenance and reinforcement of democracy in Guyana," they said.
The United States also voiced concern about the possibility of electoral fraud.
Michael Kozak, a senior official with the State Department's bureau of Western Hemisphere Affairs, said "no candidate should declare victory or be sworn in while serious questions remain about credibility" of the election.
"The people of Guyana deserve a credible election and a legitimate transfer of power, the democracies of the region expect that Guyana will uphold our shared values," Kozak wrote on Twitter.
Demerara-Mahaica, a government stronghold known as Region 4, contains the capital Georgetown and is the largest in Guyana, with 285,618 registered voters.
The figure is considered to be bloated however by a failure to remove migrants and the dead from voting rolls.
Election commission results from Region 4 published Thursday showed the ruling coalition with almost twice as many votes as the opposition People's Progressive Party, with some 70,000 ballots not yet counted.
Late Thursday, the PPP secured a court ruling compelling the commission to verify results before publishing. But the order was not served as the chairwoman and chief elections officer could not be tracked down.
The Guyanese election is being watched more closely than might ordinarily be the case because the eventual winner will be in control of a coming oil boom that is set to transform Guyana.
The International Monetary Fund expects the country's economy to record the biggest growth worldwide this year, a staggering 85 percent.