Guyana's opposition candidate was sworn in as president on Sunday, hours after being declared winner of the disputed March 2 general election in the oil-rich, cash-poor South American nation.

Irfaan Ali, 40, of the People's Progressive Party (PPP) was named the top vote-getter by Guyana Elections Commission chief Claudette Singh 154 days after the poll, following a recount, court battles and allegations of fraud.

Outgoing leader David Granger, 74, said his coalition "respects the lawful consequences of the 'declaration'" by the elections commission, but also said claims of electoral fraud and other irregularities would be taken to the High Court.

Granger appealed to his supporters to be peaceful.

Ali called for national unity in this former British colony whose population is almost evenly split among ethnic lines, with descendants of Indian indentured laborers supporting the PPP and descendants of African slaves backing Granger's coalition.

"There is only one future and that future requires a united Guyana, that future requires a strong Guyana," Ali said.

Irfaan Ali holds his son as he arrives at a campaign rally in Lusignan, Guyana on February 29, 2020
Irfaan Ali holds his son as he arrives at a campaign rally in Lusignan, Guyana on February 29, 2020 AFP / Luis ACOSTA

The election in Guyana, a country of 750,000 bordered by Brazil, Venezuela and Suriname, was being watched more closely than might ordinarily be the case because the winner will be in control of a coming oil boom.

Last December, ExxonMobil began commercial exploitation of a huge 2016 oil discovery off the coast, and production is expected to grow from 52,000 barrels per day to 750,000 from 2025.

The declaration of Ali, a former housing minister, as winner brings to an end five months of court cases by Granger's Partnership for National Unity and Alliance for Change (ANPU-AFC) over results from a national vote recount, after international observers said initial results in Guyana's most densely populated electoral district had been inflated in favor of the incumbent.

International parties including the Organization of American States (OAS), Caribbean Community (CARICOM), European Union, US, Britain and Canada had called for recount figures to be used to declare the winner.

On July 1, US Secretary of State Mike Pompeo had said Guyana needed to "get on with" a transition of power, and two weeks later imposed visa sanctions on members of Granger's administration "responsible for, or complicit in, undermining democracy in Guyana."

Under Guyana's proportional representation electoral system, the PPP won 33 of the 65 seats in the National Assembly with 233,336 votes. The APNU-AFC earned 31 seats with 217,920 votes.