Kenya's President Mwai Kibaki has been sworn in for a second term after the electoral commission declared him the winner of closely-fought elections.
Opposition leader Raila Odinga rejected Mr Kibaki's victory, accusing him of electoral fraud and demanding a recount of the results of Thursday's poll.
The commission's announcement triggered riots around Kenya.
The count was badly delayed, sparking violence in which at least 13 people are reported to have been killed.
A team of observers from the European Union said the Electoral Commission of Kenya (ECK) had failed to ensure the credibility of the poll.
"We believe that... the ECK... has not succeeded in establishing the credibility of the tallying process to the satisfaction of all parties and candidates," chief EU observer Alexander Lambsdorff said in a statement.
British Foreign Secretary David Miliband said London had "real concerns at the irregularities reported by the EU observers and others".
In Washington, US State Department spokesman Tom Casey condemned the violence.
"Regardless of the eventual winners of this election, we call on Kenyans across the political spectrum to work together to advance democracy and national development," he said.
'Time for healing'
Mr Kibaki won 4,584,721 votes, beating Mr Odinga by more than 230,000 votes, the ECK said.
Kalonzo Musyoka, another presidential contender, got 879,903 votes.
"The commission therefore declares Honourable Mwai Kibaki as the winner," ECK chairman Samuel Kivuitu said.
Mr Kibaki was then sworn in for his second five-year term - about an hour after the ECK statement.
In his speech, he described the elections as "free and fair", urging all political parties to "accept the verdict of the people".
Mr Kibaki said it was now "time for healing and reconciliation" to overcome issues dividing the nation.
He also promised to form a "clean hands government" free of corruption.
The ECK announcement immediately triggered riots in Nairobi's Kibera slum.
Police shot dead five opposition protesters in two separate incidents in Mr Odinga's strongholds in western Kenya, police were quoted as saying by the AFP news agency.
The Kenyan government suspended all live television broadcasts amid post-election violence.
Meanwhile, Mr Odinga's Orange Democratic Movement said it was planning an alternative inauguration ceremony for the party leader on Monday in Nairobi.
Mr Odinga had led since vote counting began, but saw his advantage evaporate as later votes were added.
The ECK announcement was delayed, following chaotic scenes at the commission's headquarters in Nairobi.
Just minutes after Mr Kivuitu began reading the poll results, an opposition member approached the podium, shouting "Justice!" and "This is not a police state!"
Pandemonium then broke out, as other members of Mr Odinga's party got to their feet and began shouting, the BBC's Adam Mynott reports from inside the ECK headquarters.
Mr Kivuitu had to be led away from the building by police, only to return later and announce Mr Kibaki as the winner of the poll.
Mr Odinga had earlier said that Mr Kibaki "flooded this [electoral] commission with his cronies".