As the sun set on Kisumu, large numbers of protesters congregated on a roundabout in the western lakeside city, throwing stones and setting tires on fire as they blocked roads with broken rocks.
Many shops in Kisumu had closed early, fearing possible protests over the result, and at least one supermarket was looted as young men walked away with food and electronic goods.
"It was not free and fair. We were cheated," 26-year-old Odinga supporter Collins Odoyo told AFP as he rushed off to join the crowd, barefoot and with a vuvuzela horn strapped across his back.
"We were cheated," Isaac Onyango, 24, said.
"The government must listen to us. They must redo the election," said Onyango, 24, his eyes streaming as police tried to defuse the demonstration with tear gas.
"You can't steal from us!" shouted a young man in a balaclava wielding a club.
Another protester loading a rock into a slingshot yelled: "We will not surrender!"
AFP correspondents reported that police fired live rounds as protests erupted in the Nairobi slum of Mathare where Odinga is popular.
And across town in Kibera, one of Nairobi's largest slums, supporters demanded a re-run as they hurled stones.
"Baba's vote has been stolen," said motorcycle taxi driver Emmanuel Otieno.
"Stop lying to Kenyans, we know Baba won," said another protester, Eliud Omolo, waving a banner supporting Odinga.
The dispute is likely to further damage the reputation of the IEBC after it had faced stinging criticism over its handling of the 2017 election which was annulled by the country's top court in a historic first for Africa.
William Ruto was Monday declared the victor of Kenya's hard-fought presidential poll, but the outcome sparked a split in the election commission and some violent protests in his defeated rival's strongholds.
Ruto won with 50.49 percent of the vote on August 9, narrowly ahead of Raila Odinga on 48.85 percent, Independent Electoral and Boundaries Commission chairman Wafula Chebukati announced after an anxious days-long wait for results.
With tensions running high after the disputed outcome, the 55-year-old president-elect vowed to work with "all leaders".
"There is no room for vengeance," said Ruto. "I am acutely aware that our country is at a stage where we need all hands on deck."
Four out of seven IEBC commissioners rejected the outcome of Tuesday's vote, with vice chair Juliana Cherera describing the process as "opaque".
But Chebukati, who was also in charge of the IEBC in 2017, insisted he had carried out his duties according to the law of the land despite facing "intimidation and harassment."
The victor's supporters were also vocal.
"William Ruto is going to be the president of those who lose and of those who win," 34-year-old Laban Keter told AFP.
"Let's take this country forward by keeping the peace and accepting the winner of the election," he said.
"It is a victory for all Kenya, there is no divide, we are all one family, we are all brothers, we are all sisters. Kenya is one," said wholesaler Hillary Kebenei.
The dispute will test Kenya's stability after previous elections in the East African political and economic powerhouse were blighted by claims of rigging and vicious bouts of deadly violence.
With memories of previous post-poll violence still fresh, both Odinga and Ruto had pledged to accept the outcome of a free and fair election, and air their grievances in court rather than on the streets.
Polling day had passed off generally peacefully. But power transfers are fraught in Kenya, and how Odinga handles defeat will be anxiously watched by the country's foreign partners.
No presidential ballot has gone uncontested in Kenya since 2002, and a Supreme Court challenge by Odinga is seen as almost certain.
If there is no court petition, Ruto will take the oath of office in two weeks' time, becoming Kenya's fifth president since independence.
Any challenge to results must be made within seven days. The Supreme Court has a 14-day deadline to issue a ruling, and if it orders an annulment, a new vote must be held within 60 days.
In August 2017, the Supreme Court annulled the election after Odinga rejected the results that gave Kenyatta victory, with dozens of people killed by police in the protests that followed.
Kenyatta went on to win the re-run two months later after an opposition boycott.
The worst electoral violence in Kenya's history occurred after a disputed vote in 2007, when more than 1,100 people were killed in bloodletting between rival tribes.