Election officials have been painstakingly trying to confirm their provisional results with checks of documents from polling stations nationwide.
With at least 60 percent of ballots from Tuesday's vote counted, Kenyatta has 55.3 percent, compared with 43.9 percent for his main rival, according to the Independent Electoral & Boundaries Commission's website.
Mudavadi's claims echoed similar accusations tabled by Odinga on Wednesday when he described the counting process as a "sham" and said hackers had broken into the tallying systems and interfered with the results.
Odinga had said hackers could have used the identity of a top election official, who was tortured and murdered days before the vote.
They basically gave an approval to a fairly flawed process.
"This is a fraud of monumental gravity".
Under a power-sharing deal brokered by former United Nations secretary-general Kofi Annan, Mr Odinga served as prime minister from 2008 to 2013. "We didn't have an election".
Most opposition supporters have so far heeded calls from their leaders to remain calm until an official declaration of the victor is made. "We continue to urge everyone to be calm, to be resilient and to be peaceful", said the EU's Schaake.
Speaking at a news conference, Odinga urged his supporters to remain calm, but added: "I don't control the people".
But challenger Raila Odinga claims those numbers were manipulated.
Kisumu shopkeeper Festus Odhiambo said he was praying for peace even as protesters blocked roads into city slums with bonfires and boulders. Most of the youths were outside and I saw youths have started to harass people who were just walking.
"We expect now a sense of responsibility and leadership by all parties", the European Union said in a statement on Friday.
Chebukati says "hacking was attempted but did not succeed" and that the tallying of final results is continuing. He said the opposition's hacking complaints should be looked into by the IEBC, saying this was not the mandate of observers.
"We do not want to see any violence in Kenya".
Tensions remain high in the country following rumours of election fraud in Tuesday's presidential election.
"People are not happy", said opposition supporter Oyoo Ogango, 30, who is unemployed with three children. "All we need is credible elections".
But the opposition insisted it was in possession of alternative results, showing Odinga in the lead.
"When there's some doctoring, I must be anxious".
Provisional results indicated a strong lead for President Uhuru Kenyatta.
"We call on all Kenyans and worldwide partners to continue to work together in the spirit of peace, partnership and democracy over the coming hours and days". Other ethnic groups were also drawn into the violence.
Reporting from Nairobi, Sky's Foreign Affairs Editor Sam Kiley said: 'This is the sort of election violence that was predicted and feared.
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