Kenya's President Uhuru Kenyatta has taken an early lead as results from national polls rolled in on Wednesday, the election board said, but the opposition dismissed the numbers as "fake" and said their own tally showed they were winning.

Tuesday's contest between Kenyatta, a wealthy 55-year-old businessman, and Raila Odinga, 72, a former political prisoner and son of Kenya's first vice president, was a hard-fought election that stoked fears of possible violence.

Kenyatta was leading with 55 percent of the vote and Odinga had 44 percent at 3:00 am (00:00 GMT), the election commission website said, after more than three-quarters of the 40,833 polling stations reported results.

But Odinga angrily dismissed that tally as "fictitious ... fake" at a late-night press conference and said the results were a sham because they were not accompanied by scanned copies of forms that party observers in all polling stations should have signed to certify the results.

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"We have our projections from our agents which show we are ahead by far," Odinga said, questioning why Kenyatta had maintained a fairly static lead since tallying began. He also linked his allegations of vote rigging to the unsolved torture and murder of a top election official days before the vote.

"We fear that this is the precise reason why Mr Chris Msando was assassinated," Odinga said, referring to his fraud claims.

The commission did not release information about which constituencies had been counted, so it was unclear whether Kenyatta strongholds, opposition centres or some combination had yet to be tallied from Tuesday's vote.

"A clean credible process would by now have a dashboard showing all tallies from all constituencies to add to a sum total so that the country can know which part of the country has been counted and what the votes are," Odinga said in a statement on Wednesday.

"The system has failed," Odinga said.

Raphael Tunju, secretary-general of Kenyatta's Jubilee party, shrugged off Odinga's allegations.

"I don't expect anything else from NASA," he said, referring to Odinga's National Super Alliance party.

"Let's put it this way, if the results, which are being streamed, showed that they were leading, what would they be saying now?"

Election officials acknowledged the opposition objection, but defended their actions.

"We believe that by displaying results, we have been doing well to enhance transparency and accountability in the electoral process, consistent with the commitment the commission has made to the Kenya people," said Commissioner Consalata Bucha Nkatha Maina, vice chairwoman of the election commission.

The commission's CEO, Ezra Chiloba, also said a results screen at the commission's counting centre had frozen because too much data was being received and that tallies would be updated later Wednesday morning.

Al Jazeera's Mohammed Adow, reporting from Nairobi, said Odinga, in a press conference, did not show any proof backing his allegations.

Fears of violence

Campaigning was marked by fiery rhetoric but public speeches were largely free of the ethnic hatred that has marred previous contests as the two men faced off for a second time.

Opinion polls released a week ago had put them neck-and-neck.

Odinga comes from the Luo people in western Kenya, an area that has long felt neglected by the government and resentful of their perceived exclusion from political power.

Kenyatta is a Kikuyu, an ethnic group that has supplied three of Kenya's four presidents since independence from Britain in 1963.

Any glitches - innocent or otherwise - in the hi-tech electronic voting and tallying system could be grounds for the loser to allege fraud, as Odinga did in 2007 and in 2013. Given his age, this is probably his last campaign for the top job.

A decade ago, vote tallying was abruptly stopped and the incumbent president declared the winner, triggering an outcry from Odinga's camp. The ethnic violence that followed killed 1,200 people and displaced 600,000.

International Criminal Court cases against Kenyatta and his now-deputy, William Ruto, for helping direct that violence collapsed as witnesses died or disappeared.

During the 2013 polls, Odinga again alleged fraud, but quelled unrest by taking his complaints to the courts.

This time, the government deployed more than 150,000 security personnel, including wildlife rangers, to protect 41,000 polling stations.

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Thousands of people had returned to their ethnic heartlands before the vote, fearful of a repeat of the 2007 violence.

Voting was mostly smooth and turnout was high, the election commission said, despite some isolated incidents and delays. The police said there were no major problems.

'The will of the people'

As he voted, Kenyatta said he would be willing to step down if he lost and called on Odinga to do the same.

"In the event that they lose, let us accept the will of the people. I am willing myself to accept the will of the people, so let them too," he said as he voted at the Mutomo Primary School in Gatundu, some 30km north of the capital.

Hundreds of people had waited in line since 2am (23:00 GMT), wrapped in jackets and blankets against the cold and drizzle.

The winner needs one vote more than 50 percent, and at least a quarter of the vote in 24 of Kenya's 47 counties.

There are more than 19 million registered voters in the nation of 48 million. Half are aged under 35.

There is little to separate the two candidates in terms of economic policy although the US-educated Kenyatta is seen as more pro-business.

Throughout the day, Kenya's shilling held steady at around 103 to the dollar.

Odinga voted in Kibera, Kenya's biggest slum, but left without speaking to supporters. Later, his main ally Musalia Mudavadi said the opposition alliance was largely satisfied with the vote so far.

"We also wish to commend security agencies for the professionalism displayed so far," he said in a news conference.

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The opposition, which has accused the government of trying to rig the poll, listed some problems, from agents being denied entry to polling stations, to names missing from the register and electronic voter identification devices failing.

The ruling Jubilee party also raised some issues but said it was largely satisfied.

The torture and murder last week of a top election official and the deportation of two foreign Odinga advisers have fuelled wild online conspiracy theories and "fake news" items consumed voraciously by Kenya's tech-savvy population.