The opposition candidate in the Kenyan presidential election has rejected the victory of President Mwai Kibaki, who has been sworn in for a second term.
Raila Odinga accused President Kibaki of electoral fraud
Raila Odinga claims that he lost the election because of large-scale vote rigging.
Mr Odinga's party has said it will hold an alternative inauguration ceremony on Monday to declare him president.
The election result has sparked fears of ethnic violence, with riots breaking out across the country.
Overnight, there were reports that several people had been killed in the central Kenyan city of Nakuru.
Local police chief Stephen Munguti, quoted by AFP news agency, said that seven people had been killed as a result of fighting between rival political groups.
On Monday morning, there were running battles between police and mobs armed with clubs and machetes in Kibera, a large slum in the capital.
A curfew has been imposed in the western city of Kisumu after clashes there.
Mr Odinga has planned a ceremony for Monday in Nairobi's Uhuru Park. The event has been banned by police, and hundreds of riot officers have been deployed around the park.
A team of observers from the European Union said the Electoral Commission of Kenya (ECK) had failed to ensure the credibility of the vote.
"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.
The British Foreign Secretary, David Miliband, said London had "real concerns at the irregularities reported by the EU observers and others".
But the United States congratulated Mr Kibaki on his re-election and called on all sides to respect the result despite the allegations of fraud.
President Kibaki was sworn in for a second five-year term on Sunday immediately after the ECK announced he had beaten Mr Odinga by more than 230,000 votes in Thursday's closely-fought election.
In a speech, Mr Kibaki described the elections as "free and fair" and urged all political parties to "accept the verdict of the people".
He said it was now "time for healing and reconciliation" to overcome issues dividing the nation and promised to form a "clean hands government" free of corruption.
Earlier on Sunday, Mr Odinga had called on the president to concede victory and demanded a recount, saying the ECK "cannot possibly address the multiple levels of fraud administered by this administration".
"There is a clique of people around Kibaki trying to rob Kenyans of the election," Mr Odinga later told his supporters after the results were announced.
"The train of democracy in Kenya is unstoppable like the flow of the Nile."
As Mr Odinga spoke, the government suspended all live television and radio reports, ending the transmission of his news conference.
"People who want to commit a crime do so in darkness," he said afterwards.
Mr Odinga's Orange Democratic Movement later invited its supporters to an alternative inauguration ceremony at a park in central Nairobi on Monday afternoon to declare him the "people's president".
"We know that the people of Kenya elected Raila Amolo Odinga as their legitimate president and they are ready to see him serve democratically in the capacity," it said in a statement.
After the ODM's announcement, Kenyan police warned Mr Odinga that he would face arrest if he went ahead with the demonstration and risked inciting violence.
Soon after the official results were declared, thousands of opposition supporters took to the streets across the country in protest.
The announcement of the election result sparked riots across Kenya
At least 13 people were killed in the third day of unrest that has broken out in Kenya since the election, police said.
The worst violence occurred in the western province of Nyanza, where Luo supporters of Mr Odinga clashed with members of Mr Kibaki's Kikuyu tribe.
Ten people were killed in the ethnically-mixed town of Kisii, local television stations reported, while police shot into a crowd in Kisumu, killing another three people.
In Nairobi, police fired live rounds and tear-gas to disperse Mr Odinga's supporters in the Kibera slum, while a blackout also plunged the area into darkness.
"There can be no peace without Raila," said one resident.
The southern coastal city of Mombasa was also affected, with a post office and a restaurant set on fire by demonstrators. There were also reports of trouble in Bungoma, Busia, Eldoret, Kericho and Kakamega.
Large numbers of paramilitary police have been put on stand-by by the government.