Diplomats and foreign observers on Thursday questioned preliminary results of Guyana’s presidential election as incumbent David Granger claimed victory and the opposition denounced fraud, setting up a showdown in the oil-rich South American nation.

The former British colony on Monday voted to choose who will oversee an oil boom that promises to be transformative for the poor country thanks to recent crude discoveries off its coast.

Opposition leaders denounced “thievery,” and diplomats from the United States, the European Union, Britain and Canada said results released by the elections council for one of the country’s regions were not credible.

“This is an embarrassment that this emerging oil giant has to be sitting here at this table now and speaking on the brink of a dictatorship,” said Kian Jabour of opposition party A New and United Guyana.

A disputed election could derail the country’s plans to use its newfound oil wealth to spur economic development. It would fuel long-simmering ethnic tensions between the country’s Afro-Guyanese and those of Indian descent, who have grown suspicious that the other is seeking to control oil revenue.

Opposition leaders said the elections commission altered results from an area called Region Four, the most populous electoral district, to give Granger, the sitting president, of the APNU-AFC coalition, a wide margin over Irfaan Ali of the opposition PPP party.

Former President Bharrat Jagdeo said the commission’s results for Region Four did not match the sum of votes tallied in the statements of poll – the official documents that reflect voting results at each individual polling station.

“Based on our observation of today’s (election’s commission) proceedings at their Region 4 office, and the fact that the full count was not completed, we question the credibility of the Region 4 results,” the heads of the four diplomatic missions said in a joint statement.