The Coalition for Reforms and Democracy (CORD) alliance, which alleges that the electoral body is biased towards the president, has been staging weekly protests every Monday.
Protests took place in several towns but in Kisumu, an opposition stronghold in western Kenya, police opened fire before using tear gas and water cannon to quell the protests.
Word of the shootings fuelled heavy clashes in the centre of Kisumu and the working class district of Kondele. There were widespread scenes of looting and two supermarkets were destroyed.
A bullet wound was visible on one corpse, laid outside a hospital morgue by protesters who said he was hit by police fire.
Six protesters were taken to Kisumu's main hospital suffering from gunshot wounds, the Red Cross said.