Violence raged across Kenya for a second day as supporters of opposition candidate Raila Odinga clashed with police over a presidential vote they say has been stolen. Nick Wadhams reports from Nairobi.

Looters in Mombasa, 31 Dec 2007

U.N. Secretary-General Ban Ki-moon has appealed for calm in Kenya where post-election violence has killed at least 150 people.

Ban issued a statement late Monday deploring the loss of lives and calling on security forces to show restraint in dealing with protesters. The U.S. State Department made a similar appeal.

Both the opposition cabdidate Raila Odinga and President Mwai Kibaki appealed for calm, but Odinga said he had strong evidence that Kibaki's team had rigged the vote, padding the tallies in several areas with phantom ballots.

On Sunday, the country's electoral commission announced Mr. Kibaki won by just 230,000 votes.

Raila Odinga gives a press conference in Nairobi after cancelling a rally, 31 Dec 2007

Odinga decided to cancel a Monday rally in downtown Nairobi, after the government refused to allow it and ringed the protest site with police. Instead, he planned a day of protest for Thursday and urged his supporters to wear black armbands as a sign of opposition to Kibaki's win.

"I am the elected president of the Republic of Kenya and that is the role that I want to exercise if Mr. Kibaki would allow me to do so," he said. "But if Mr. Kibaki does not want to do so, the people of Kenya will make him do so."

The violence wracking Kenya has thrown the country into its biggest crisis since independence from Britain in 1963. With Odinga in no mood to concede, the danger could be of a wider ethnic conflict between Luos, who back Odinga, and Kikuyus, who support the president.

Police officers detain an opposition supporter during riots at the Kibera slum in Nairobi, 31 Dec 2007

While much of the violence has been between protesters and police, people say they have been targeted in the rising tension. Peter Mwau, who lives in the Kibera slum, comes from the small Kamba tribe, and said he was fearful of harassment by Odinga's Luo supporters.

"They took my money, what I had in my pocket, then I just went," he said. "I did not even talk to them. We did not even sleep. They were just breaking into the shops. What they did bad is just breaking into the shops, then they looted everything."

Fears that the vote may have been rigged appeared to be confirmed by election observers.

Residents of the Kibera slum, the largest in Nairobi, march past a burning house during post-election riots, 31 Dec 2007

The European Union said it too believed there was evidence of manipulation, while a domestic observer group, KEDOF, said the electoral commission never addressed any questions before it went forward with the results.

The third presidential candidate, Kalonzo Musyoka, called for peace. He said the results should be accepted, despite the concerns from the Odinga camp.

"It is not fair to just blame it on President Kibaki," he said. "We would rather accept that these results were less than satisfactory. But we have to get on with life after this. And the next thing in a country that upholds a principle of law and democracy is to move to court, and we as a party will be doing that, but our only fear is that unless there is a reconstituted judiciary, we may have problems."

The day also saw a reversal from the U.S. State Department, which said late Sunday it accepted the results and congratulated Mr. Kibaki on his re-election. But Monday, the Embassy in Nairobi issued a statement saying it was concerned by "serious problems experienced during the vote-counting process."