GEORGETOWN, Guyana — Guyana’s opposition party has won a bitterly contested general election, ending a prolonged political standoff that had crippled investment and heightened ethnic tensions in the small South American nation.

The opposition candidate Mohamed Irfaan Ali was sworn in on as Guyana’s president on Sunday, shortly after the national electoral commission said he had beaten the incumbent, David Granger, by just over 15,000 votes, a margin of more than 3 percent. The governing party said it planned to challenge his victory, alleging fraud.

As president, Mr. Ali will manage billions of dollars in new oil revenues, which have transformed Guyana, an impoverished former British colony with fewer than 800,000 people, into the world’s fastest-growing economy this year, despite a slumping global oil market.

Mr. Ali’s assumption of office follows five months of political wrangling between Guyana’s two major political parties over the outcome of the March 2 vote, which exposed deep tensions between Black citizens and those of South Asian descent. The power struggle has been amplified by the newfound wealth pouring in from offshore oil fields where production began in January.

The election dispute brought a 100-day ballot recount, at least half a dozen court cases and accusations of fraud against both major parties.

After the recount showed Mr. Ali, 40, winning a slim victory, the governing party’s allies in the electoral commission repeatedly tried to present results that did not match the recount tally approved by electoral observers.

Members of the governing party also flooded the courts with petitions for injunctions to block the declaration of results, and the party’s activists threatened to make the country ungovernable if Mr. Ali was declared winner.

Supporters of both political parties, which are split almost exactly among ethnic lines, fear they will be excluded from the oil bounty if their opponents take power.

Growing international pressure and economic pain appeared to persuade the governing party to cede control. Mr. Granger’s intransigence was condemned by practically all of Guyana’s economic partners, including its usually restrained Caribbean neighbors and the United States, which revoked visas of top government officials for subverting the elections.

Mr. Granger, 75, continues to assert that a quarter of all the votes showed signs of irregularities, although the results were endorsed by at least four major groups of international observers.

In a statement on Sunday, Mr. Granger said that he “cannot endorse a flawed report,” and that he would continue to “campaign to ensure that the votes of all Guyanese are accurately recorded.” He added that he would challenge the results in court, but stopped short of calling supporters into the streets.

Guyana’s prolonged political standoff had deepened the economic pain of the pandemic, which is ripping through South America. Despite a steep rise in oil production, local businessmen said their international partners had delayed investments and banks had cut credit until the political crisis was resolved.

“These past five months were very hard: People lost their jobs, money wasn’t circulating,” said Nasrudeen Alli, a mechanic from a suburb of the capital, Georgetown. “I am very optimistic that we can move forward from this shadow and into the light of betterment for all.”

The legal limbo had prevented Mr. Granger’s government from spending the accumulating oil revenues and mobilizing emergency funds to combat the coronavirus and the ensuing economic fallout. Without an approved national budget, Guyana’s public spending shrank to a minimum.

The electoral crisis has also brought fears of a return of the ethnic violence that had plagued Guyana’s politics for decades. Mr. Ali’s swearing-in ceremony was greeted cautiously by supporters, who are mostly of South Asian descent, for fear of provoking retribution from Mr. Granger’s primarily Black followers.

“All I am asking for right now is for us to get back to normalcy, where we can reap the benefits of a functioning government,” said Ramsook Sukdeo, a house painter from a coastal village of Bush Lot.