From early morning, hundreds of youths took to the streets of Kisumu - a normally sleepy city on the shores of Lake Victoria - burning tyres, ransacking shops and blocking roads.
Residents said at least one person had died.
In Nairobi's Kibera shantytown, also a hotbed of Odinga support, locals said two people were killed in skirmishes and police deployed as rival ethnic gangs faced off.
Residents said trouble began in the sprawling slum - one of Africa's biggest - before dawn. Shots were fired, and numerous shacks torched. Armed police stood between two gangs, one Luo, the other Kikuyu, who were brandishing knives and clubs.
In Kisumu, and other pro-opposition western areas, looters targeted Kikuyu businesses.
"We have just started. We will loot all Kikuyu shops and kill them on sight," said Richard Ondigi, 23, a driver.
One crowd waving machetes yelled "Death to Kikuyus" as youths carried off stolen goods including furniture and crates of drinks. Young boys swigged looted beer.
In Nairobi, streets were near-deserted in the city centre as business owners pulled down shutters on their stores.
Truckloads of military police poured in to patrol.
If Odinga wins, he would realise a long-held ambition to rule Kenya - a dream that eluded his late father, nationalist hero Jaramogi Oginga Odinga who became vice-president.
The inauguration of Kenya's presidential winner will take place within days.
Should it be Odinga, his priority will be to enlist support of the economically powerful Kikuyus, ensure a peaceful handover, and allay business fears he is a left-wing radical.
Educated in East Germany and with a first-born son called Fidel Castro, Odinga has throughout the campaign sought to quash anxiety he may have a hidden socialist agenda.
The ECK forecast record turnout figures for what became Kenya's tightest race since British colonial rule ended in 1963.
