The former United Nations leader Kofi Annan warned Kenya's leaders to tone down their rhetoric as the coalition of defeated presidential candidate Raila Odinga claimed more than 100 people had died in post-election violence.
A day after Kenyatta was declared the victor of the presidential poll as police opened fire to disperse protesters who blocked roads and set up barricades in the capital, Nairobi, on Saturday.
Kenyan police have killed at least 11 people in a crackdown on protests as anger at the re-election of President Uhuru Kenyatta erupted in the western city of Kisumu and slums surrounding the capital, officials and witnesses said on Saturday.
Kenya elections have routinely been marred by violence since the country became a multiparty democracy in 1991.
Kenyatta was declared the victor of a disputed poll on Friday, beating his long-time rival Raila Odinga by a huge margin. Katana says another five people were injured by gunfire in Kisumu.
And Kenyatta extended an olive branch to his opponents.
He later visited Mathare slum to console the family whose 10-year-old daughter was killed by a stray bullet.
He also condemned police brutality and killings in Kibera and Mathare, saying that Nasa leaders had been aware of plans to do so before the election. At a press conference after that happened, they said, they were ambushed by security services. "There's no secret about that", Odinga said.
Politics in Kenya is largely divided along tribal lines, and the winner-takes-all nature of elections has long stoked communal tensions.More news: Nikkei flat as North Korea fears recede; financial stocks tumble
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UN Secretary-General Antonio Guterres on Saturday called on Kenyan opposition leaders to address their disputes over the presidential election through legal means instead of violence.
Protests in Mathare following the declaration of President Uhuru Kenyatta as the victor at the polls. The opposition called the election process a "charade".
The announcement of the final tally was delayed by seven hours and took place without representatives of the National Super Alliance (NASA) opposition coalition being present. "We want peace but we want peace with justice", said Orengo.
Kenyan Acting Interior Minister Fred Matiang'i denied that police fired live bullets and said there were no real protesters, just criminals.
The official results gave President Uhuru Kenyatta 54.3%, and Mr Odinga 44.8%.
Kenyans have anxious about the unrest that might follow this year's elections.
The global Elections Observation Group, which deployed 8,300 observers, said its projected outcome for the results had Kenyatta wining with 54 percent of the vote, 0.3 percent shy of the official figure.
Around 1,200 people were killed then and 600,000 displaced after Odinga called for political protests that sparked ethnic violence. As the largest economy in East Africa, Kenya is a crucial trade route to the continent and provides an important buffer of stability in a region that includes the fledgling Somali government and the politically tense Sudan and South Sudan.