NAIROBI (AFP) - Kenya's outgoing Prime Minister Raila Odinga, who was narrowly defeated in presidential polls by Mr Uhuru Kenyatta, filed a formal challenge on Saturday against the result.

Officials from Mr Odinga's Coalition for Reform and Democracy (CORD) filed the suit at the Supreme Court after Mr Odinga spoke to supporters and journalists in front of his offices.

"I have no hesitation whatsoever in lawfully challenging the election outcome," he said. "To do otherwise would be a betrayal of the new Constitution and, therefore, of everything that Kenyans hold dear."

Mr Kenyatta, who avoided a second-round run-off vote by the slimmest of margins to win a majority with just 50.07 per cent, beat Mr Odinga - his closest rival - by more than 800,000 votes.

Mr Odinga won 43.31 per cent of the votes in his third failed attempt at the top job. He said he would abide by the decision of the court and urged Mr Kenyatta to do likewise.

"I have repeatedly indicated my commitment to respect and abide by the Supreme Court ruling. I invite my brother Uhuru to publically do the same," he said.

Party officials had been due to file their petition on Friday but said they were delaying the case until Saturday - the final day allowed - as civil society groups were expected to first submit separate petitions.