Both candidates insisted they were winning the election, which marks the first time an incumbent has faced a credible challenge in Kenya's four decades of independence from Britain.
By Saturday afternoon, the Electoral Commission said millionaire opposition candidate Raila Odinga was leading with 3.7 million votes to President Mwai Kibaki's 3.4 million, with 159 of the 210 constituencies counted.
"We are confident that (Odinga) has won the election," said his campaign manager, Mohamed Isahakia.
President Mwai Kibaki's Party of National Unity also said it was winning.
"We in PNU have added all our figures and we are pleased to announce that Honorable Mwai Kibaki is winning this year's election," said Noah Wekesa, Kenya's minister for science and technology.
There was potential for even more delays, however, due to the complicated rules for winning. A presidential candidate has to get the most votes as well as garner at least 25 percent of votes in five of Kenya's eight provinces, a move aimed at ensuring a president has some support in most of the country and its many tribes.
The trickle of results ignited tensions in the capital and opposition strongholds Saturday.
In the Kibera slum, Odinga's main constituency, young men with fingers still stained with voting ink were shouting "No Raila, No Kenya!"
- an ominous call to declare him the winner. Hundreds of people swarmed out of the slum, heading for town, but police used tear gas to chase them back.
Smoke was billowing out of Kibera as homes, trees and stalls caught fire.
Hamisi Noor, 22, who was standing in front of his burned-out home in Kibera, said a crowd threatened him with machetes before setting his home on fire and cutting his father across the face.
Noor, a member of Kibaki's Kikuyu tribe, said his assailants belonged to Odinga's Luo tribe. "I don't know who they were," said Noor, his trousers covered in blood and mud. "But they were Luos."
Police were blocking off streets as young men climbed up billboards to rip down Odinga posters in the capital of Nairobi, about 6 miles outside the deserted city center.
"Kibaki come back!" the men shouted as they waved machetes and sticks.
There were no immediate reports of serious injuries.
U.S. Ambassador Michael Ranneberger appealed for calm.
"Give this process a chance to be finished," he said. "This is the time for Kenyans to come together."
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