Kenya's Supreme Court has nullified President Uhuru Kenyatta's election win last month, ruling it unconstitutional and calling for new elections within 60 days.
Mr Kenyatta said he "personally disagrees" with the ruling but respects it, although he lashed out at the judges, saying that "six people have decided they will go against the will of the people".
He also called for peace in a country where some elections have been followed by deadly violence.
No presidential election in the East African economic hub has ever been nullified.
Opposition members danced in the streets, revelling in the setback for Mr Kenyatta, the son of the country's first president, in the long rivalry between Kenya's leading political families.
"It's a very historic day for the people of Kenya and by extension the people of Africa," opposition candidate Raila Odinga, who had challenged the vote, said.
"For the first time in the history of African democratisation, a ruling has been made by a court nullifying irregular election of a president. This is a precedent-setting ruling."
The six-judge bench ruled 4-2 in favour of the petition filed by Mr Odinga.
He claimed the electronic voting results were hacked into and manipulated in favour of Mr Kenyatta, who had won a second term with 54 per cent of the vote.
"A declaration is hereby issued that the presidential election held on August 8 was not conducted in accordance to the constitution and applicable law, rendering the results invalid, null and void," Chief Justice David Maraga said.
The court did not place blame on Mr Kenyatta or his party.
It said the election commission "committed illegalities and irregularities... in the transmission of results, substance of which will be given in the detailed judgment of the court" that will be published within 21 days.
Mr Odinga called for the election commission to be disbanded and said the opposition will ask that electoral officials be prosecuted.
The lead counsel for the president, Ahmednassir Abdulahi, told the court that the nullification was a "very political decision" but said they will live with the consequences.
Mr Odinga's lawyer had asked the court to invalidate Mr Kenyatta's win, saying scrutiny of the forms used to tally the votes had found anomalies that affected nearly five million votes.
The electoral commission had said there was a hacking attempt but it failed.
International election observers, including former US secretary of state John Kerry, had said they saw no interference with the vote.
"Right or wrong, the Supreme Court has spoken. So what remains is a fresh opportunity for the people of Kenya, in exercise of their sovereign authority, to once again restate with clarity who they want as their president," electoral commission lawyer Paul Muite said.
Two dozen countries including the US, which had already congratulated Mr Kenyatta on his victory, issued a joint statement saying the court's ruling "demonstrated Kenya's resilient democracy and commitment to the rule of law".
Mr Odinga, a longtime opposition candidate and the son of Kenya's first vice-president, had unsuccessfully challenged the results of the 2013 vote that Mr Kenyatta won.
At first, Mr Odinga's supporters said they would not go to court this time but then filed a petition two weeks ago.
Kenya had been braced for further protests as the court prepared to rule on the opposition's challenge, with police deployed to sensitive areas of the capital Nairobi and streets near the court barricaded.
Human rights groups have said police killed at least 24 people in unrest that followed the August 8 vote.
Instead, opposition supporters exploded in celebration.
"This has shown all [election] observers did not do their job. We want an apology," John Wekesa, who was dancing outside the court said.
Unease around the election rose when the official who oversaw the electronic voting system was found tortured and killed days before the vote.
But the unrest following the vote was far calmer than the post-election violence a decade ago that left more than 1,000 people dead.
"We are not at war with our brothers and sisters in the opposition because we are all Kenyans," Mr Kenyatta said on national television.
But he added: "Five or six people cannot change the will of 45 million people."
