However, he admitted on Thursday that there had been an attempt to tamper with the poll system, but the hackers had not succeeded.
Kenyatta took to the stage to thank Kenyans "for the confidence they have shown in me".
"Violence must never be an option", Godec says.
Supporters of Kenyan opposition leader Raila Odinga are rioting in a Nairobi slum and in Kisumu, a city where he has strong support. He vowed to "continue the work that we have already started and re-dedicate ourselves toward serving this great nation and our people".
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Wafula Chebukati, chairman of the electoral commission, rejected the opposition's evidence of election fraud in a letter released Friday morning, calling it "obviously and plainly falsified".
The coalition said Odinga, 72, won the election and said his supporters wouldn't accept the result. Protecting their independence, too, is essential.
In his victory speech, Mr Kenyatta said there was no need for violence as he called for Kenyans to come together.
Mbeki's mission joins the European Union observer team in endorsing the elections. "This time we caught them", he tweeted.
Yakub Guliye, election commissioner in charge of information technology, said the opposition had not made a formal request and it would not act on a verbal request.
U.S. Ambassador Robert F. Godec said any disputes should be dealt with through legal channels.
"Now is the time for leaders across the political spectrum to demonstrate their commitment to that Constitution and to the institutions it creates and the values it sets out".
Before the results were read, one of the leaders of the opposition Nasa alliance, Musalia Mudavadi, said concerns about the poll had not been adequately addressed.
The opposition rejected the results even before they were declared.
Worldwide observers on Thursday gave thumbs-up to the election, and US ambassador Robert Godec issued a statement on behalf of the diplomatic community calling for the election commission to be given time to complete its work.
At least three people have died in the violence.
The current President Kenyatta garnered 8.2 million votes or 54.27 per cent of the total votes against rival Raila Odinga's 6.76 million or 44.74 of the total votes, ending a protracted four-day standoff over the outcome of Tuesday's vote, Xinhua reported.
The statement from the independent organization included several examples of discrepancies between the preliminary results on the IEBC website and the original paper copies signed by party agents at various polling stations.
After the mysterious murder and torture of a high-ranking elections official late last month, Odinga claimed that - in Eyder's words - "a hacker used that man's credentials to go into the electoral reporting system and changed the result".
Some of Kenyatta's critics see him as a son of privilege, who attended Kenya's top private schools before enrolling at Amherst College in MA. The IEBC is the only body legally allowed to count votes.
He has urged the IEBC to conduct the tallying process in an effective and transparent manner within the legally stipulated timeframe and, where necessary, to engage in dialogue with political actors before the official announcement of results.