Kenya's defeated presidential contender Raila Odinga has filed a petition at the Supreme Court challenging the result of the election.
The outgoing prime minister refuses to accept the slim first-round win by Uhuru Kenyatta in the 4 March vote.
Mr Kenyatta comfortably beat his opponent in terms of the number of votes won, gaining 50.07% to 43.28% for Mr Odinga.
However he only narrowly avoided a run-off, winning just 8,100 votes more than the 50% needed to be declared the winner outright.
The tight nature of the win has given Mr Odinga's allies confidence that they can force a run-off through the courts, though the petition calls for the whole process to be declared null and void, Reuters reports.
By the time the petition was filed in the early afternoon, hundreds of Odinga supporters gathered outside the court, many wearing T-shirts with slogans such as "democracy on trial".
His attempt to nullify Mr Kenyatta's victory will be the first significant test for Kenya's new Supreme Court, established under a constitution adopted in a 2010 referendum.
Chief Justice Willy Mutunga, appointed in 2011 to reform a legal system accused of serving the interests of the elite, will be under pressure to deliver a transparent verdict in a country where political tensions are more tribal than ideological.
Mr Kenyatta has been indicted by the International Criminal Court over the violence in 2007, triggered by a disputed election result, in which more than 1200 people were killed.
The latest vote was largely peaceful by contrast, and Mr Kenyatta declared it "free and fair" in his acceptance speech last Saturday, though he added that the electoral process could be made more refined and efficient in the future.