Raila Odinga alleged on Thursday that dozens of Kenyans have been killed by the security forces and "nobody is talking about it. Yet they have the audacity to come and advise us to forget and move on."
He spoke outside a morgue in the capital Nairobi while accompanying families collecting bodies of the police's alleged victims.
A US Embassy statement on Wednesday called Odinga's planned protest event on December 12 "extra-constitutional" and urged him to cancel the event.
Odinga successfully challenged Kenyatta's August re-election but then boycotted a rerun vote in October, demanding electoral reforms. Kenyatta was inaugurated last week.