Kenya's outgoing Prime Minister Raila Odinga, defeated in last week's presidential polls by Uhuru Kenyatta, was on Monday preparing his Supreme Court appeal alleging fraud, officials said.
The March 4 general elections -- for the presidency, regional governors, senators, members of parliament as well as local councillors -- were marred by allegations of vote rigging.
Defeated candidates have also raised concerns about the widespread failure of an electronic voter registration system -- designed to limit fraud -- as well as of the electronic transmission of results, which forced a manual tallying process.
Odinga is expected to request a "forensic audit of manual and electronic data", including investigations into the alleged "drastic reduction and rise" of votes in some constituencies after the official register was closed, a close aide to Odinga said.
His team has complained that they are being blocked from accessing data from the Independent Electoral and Boundaries Commission (IEBC).
"We have demanded certain information from the IEBC and it is not forthcoming, yet it is fundamental to our petition," Mutula Kilonzo, an Odinga team member and former justice minister, told reporters.
Despite concern over the risk of renewed violence ahead of the election, no major incidents have been reported in recent days.
Kenyatta, who avoided a second round run-off vote by the slimmest of margins to win a majority with just 50.07 percent, beat Odinga -- his closest rival -- by more than 800,000 votes.
Odinga, who won 43.31 percent of the votes, his third failed attempt at the top job, has said he will respect the decision of the Supreme Court even if it rules against him.
He has called for his supporters to allow the legal process to run its course, warning shortly after Kenyatta was declared president-elect on Saturday that "any violence now could destroy the country forever".
Kenyatta, one of Africa's richest men, faces trial for crimes against humanity at the International Criminal Court (ICC) over his alleged role in orchestrating the bloody violence that followed the previous elections in 2007.
More than 1,100 people were killed and over 600,000 fled their homes in the post-poll unrest, one of the worst bouts of ethnic bloodletting in Kenya.
The peaceful nature of the March 4 election has been praised by the international community, which has also urged that disputes over the results be dealt with through the courts.
Kenya's neighbours as well as China have offered congratulations directly to Kenyatta, with Beijing on Monday saying it wanted to "strengthen cooperation with the new government", foreign ministry spokeswoman Hua Chunying said.
African Union commission chief Nkosazana Dlamini Zuma on Monday saluted the "people of Kenya for the successful and peaceful" elections, and congratulated Kenyatta and his running mate William Ruto, who also faces an ICC trial.
But Western nations -- who have a policy of only having essential contact with those charged by the ICC -- have praised the Kenyan people while avoiding mention of Kenyatta's win.
Odinga legally has seven days to make a petition, with the Supreme Court then having 14 days to make their ruling.
Kenya's Chief Justice Willy Mutunga has the power to order a recount, order a whole new election or dismiss the petition.
"We are going to listen to all cases brought before us fairly," Mutunga said Monday.
If the petition is dismissed, Kenyatta would be inaugurated as Kenya's fourth president one week later, suggesting a possible ceremony in early April.
Kenyatta has offered "my older brother" Odinga an olive branch, telling thousands of his loyalists he wanted to work with him "in moving our nation forward."
Kenyatta's trial at the ICC opens on July 9, while Ruto's begins on May 28.
All charges were dropped Monday against Kenyatta's co-accused, Kenyan civil servant Francis Muthaura, after a key witness "recanted a crucial part of his evidence" and admitted taking bribes, ICC chief prosecutor Fatou Bensouda said in The Hague.
Some witnesses were refusing or unable to provide vital evidence, she added.
"The witnesses that had conveyed evidence have been killed or died, others refuse to speak to the prosecutor," Bensouda said.
But she said the case against Kenyatta would press ahead.