NAIROBI — In a historic ruling and a first in Africa, Kenya’s Supreme Court nullified Friday the re-election of a sitting president, ordering a new vote to be held within 60 days after finding that the balloting last month had been tainted by irregularities.

The election on Aug. 8 was conducted peacefully and was largely praised by international observers. But David Maraga, the court’s chief justice, declared the result “invalid, null and void’’ after siding with the opposition, which had argued that the vote had been electronically manipulated to assure a victory for President Uhuru Kenyatta.

Kenyatta, 55, was re-elected with 54 percent of the vote, easily surpassing the 50 percent threshold needed to avoid a runoff.

His main challenger, Raila Odinga, 72, who had petitioned the Supreme Court to nullify the election, received about 44 percent, a difference of about 1.4 million votes.

The decision came as a surprise, even to Odinga and his supporters.

A top election official in charge of voting technology was killed about a week before the election, and although the casting of ballots went smoothly, the electronic transmission of voting tallies was flawed, leading the opposition to assert that as many as 7 million votes had been stolen.

The Independent Electoral and Boundaries Commission, which was in charge of the vote, “failed, neglected, or refused to conduct the presidential election in a manner consistent with the dictates of the constitution,’’ the court said.

The six-judge Supreme Court panel found no misconduct on the part of the president, Kenyatta, but it found that the commission “committed irregularities and illegalities in the transmission of results’’ and unspecified other issues.

“Irregularities affected the integrity of the poll,’’ Maraga told a stunned courtroom.

A team of international observers, including former US secretary of state John Kerry, had said on election day that they had seen no interference with the vote. The Carter Center said Friday that Kerry’s mission had noted that althoug polls functioned smoothly, ‘‘the electronic transmission of results proved unreliable.’’

A new vote means that candidates will have to start campaigning again and possibly raise millions of dollars: Elections in Kenya generally cost about $1 billion, including spending by the candidates during the campaign and by the government to hold the election.

Thousands of people in the opposition strongholds of Kisumu, Mombasa, and parts of Nairobi streamed into the streets and whooped with joy after the news was announced, while supporters of Kenyatta in Gatundu, his hometown, were subdued.

“I am happy to be Kenyan today,’’ said Odinga, a former prime minister now in his fourth run for the presidency. “It is a historic day for the people of Kenya, and by extension the people of Africa.’’

The National Democratic Institute, a nonpartisan organization that supports democratic institutions and practices worldwide, said that it was the first example in Africa in which a court nullified the reelection of an incumbent.

Kenyatta said he respected the ruling and called on all Kenyans to respond peacefully, but he also made clear his anger toward the court.

“Millions of Kenyans queued, made their choice, and six people have decided that they will go against the will of the people,’’ he said.

Security had been increased Friday in opposition strongholds amid concern that a ruling in favor of either side could provoke protests or worse. Kenya experienced postelection violence after presidential votes in 2007, 2013, and last month, when at least 24 people were killed.

“My concern is that no matter what the court says, the losers will react violently,’’ said John Campbell, a senior fellow at the Council on Foreign Relations, before the ruling.

Immediately after the court’s announcement, however, the atmosphere was more of joy than fear, and there were no immediate reports of violence in strongholds on the losing side, including Gatundu. There, supporters were seen carrying mock coffins with the words “RIP Jubilee’’ painted on the sides, referring to Kenyatta’s Jubilee Party.

The election controversy hinged on two paper forms that legally validate the ballots — one from each of the country’s 40,883 polling stations and the other from 290 constituencies.

Representatives from rival parties were required to approve the forms before they were scanned and electronically transmitted to a national tallying center in Nairobi, where they were to be put online immediately so they could be cross-checked.

But the electronic system — which had been overseen by Christopher Chege Msando, the election official who was killed — broke down. Therefore, only the results, not the forms, were sent to the national tallying center, often by text message.

The international observers were quick to praise the electoral body after the vote, saying there was no evidence that the votes had been tampered with at polling stations and that the paper forms would show clearly who had won.

The observers assumed the forms would be easily verifiable and would be matched with figures texted to the tallying center by party officials.

But when Kenyatta was initially declared the winner, just hours after voting ended, almost none of the forms from the polling stations were online. A couple of days later, the commission said about 10,000 forms were unaccounted for.