In Nairobi, thousands of opposition supporters tried to force their way out of the city's slums as police fought back with tear gas and baton charges.
Buildings were set alight and shops were looted in Nairobi's slums.
Smoke rose over the Kibera slum as truckloads of heavily armed police were brought in to try to contain the demonstrators.
Residents said that ethnic gangs were roaming alleyways seeking to avenge members of their tribe killed in overnight violence and setting homes on fire.
Al Jazeera's Mohammed Adow reports from inside the Kibera slum
"There is fear, there is sadness, there is depression, there is also death," Andrew Simmons, Al Jazeera's Africa bureau chief, said.
"The numbers we are getting now are well in excess of 150 across the country, but I think it will be more than that when we get confirmation of various incidents.
"It isn't just here in Nairobi, it's in Mombasa, it's in Kisumu, the third city, it's in Eldoret, there's ethnic clashes there tonight. It's in many, many cities thoroughout the country."
Amos Kimunya, Kenya's finance minister, told Al Jazeera that the security forces were working to protect the innocent.
"There is a responsibilty by the government to protect, to protect public property, to protect private lives of citizens," Kimunya said.
"I believe that we are doing our best to ensure that there is very little in terms of harm being caused, either by those who are causing harm or by the security operation."
The vote has ignited resentment between Kenya's two largest tribes, with supporters of Raila Odinga, a Luo who officially came in second in the presidential vote, clashing with members of Kibaki's Kikuyu.
The head of Kenya's Red Cross said many of those killed since the December 27 election had died in ethnic violence.
"Why are we burning these shops?" Abdi Ochieng asked as he watched his Luo neighbours cart away looted sheets of corrugated iron from Kikuyu businesses.
"Kibaki does not own them. Neither does Odinga."
Odinga again demanded that Kibaki step down, saying: "I am the elected president of the Republic of Kenya."
Odinga cancelled a rally planned for Monday after police warned against it, but announced that another would go ahead on January 3 in Nairobi's Uhuru Park.
"For the last 48 hours the people of Kenya have seen their nascent democracy shackled, strangled and finally killed," he said.
|Many businesses in Nairobi's slums have|
been looted and set on fire [AFP]
Kibaki's Party of National Unity hit back, accusing Odinga's supporters of engaging in "wanton rigging by ballot-stuffing" that had fraudulently given them more than 900,000 votes and turnout of more than 100 per cent in Odinga strongholds.
Kibaki took 4.58 million votes to Odinga's 4.35 million, but the results were marred by accusations of multiple voting, disappeared returning officers and "doctoring".
"The tallying process lacks credibility," Alexander Graf Lambsdorff, the chief European Union monitor, told Reuters news agency.
The independent Kenya Election Domestic Observation Forum said "the electoral process lost credibility towards the end with regard to the tallying and announcement of presidential results".