"We will not be cowed, we will not relent," National Super Alliance (NASA) official Johnson Muthama told the media, describing a police crackdown on protests as an effort to force the coalition "into submission".
Muthama claimed that some 100 people had been killed, although this could not be independently verified.
He said Kenyatta, who the electoral commission said beat NASA candidate Raila Odinga with 54 percent of the vote "does not have any electoral mandate to be president of Kenyans."
"We will communicate our next course of action at the appropriate time. For now, we appeal to our supporters and Kenyans to stay out of harms way.