The victims included a 9-year-old girl, according to the Kenya National Commission on Human Rights.
Relatives said the victims were shot dead in various parts of the nation, including 17 in the capital of Nairobi, the group said.
Kenyatta, the 55-year-old son of the country’s founding President, was declared the winner Friday, defeating veteran opposition leader Raila Odinga for a second five-year term.
He got 54% of the vote while Odinga, a longtime rival, received 45%.
Protests and violence broke out after he was declared the winner.
Most of the demonstrators were supporters of Odinga, who has rejected the results of the presidential election, calling the vote rigged. National election officials have decried the accusations, saying the vote was free and credible.
Protests erupted in and around Kisumu, a stronghold of Odinga. One person was killed overnight in Kisumu, police said.
Violence also broke out in Nairobi’s slum of Mathare, where tear gas and gunshots rang out all night, witnesses said.
Stabhay Mokaya, 9, was killed by a stray bullet during protests in Mathare early Saturday, her father said.
“There were three kids playing in the balcony on the fourth floor, and then all of a sudden there was a gunshot which hit the young girl,” Wycliff Mokaya said.
Mokaya said police were surprised to find out a bullet had hit the balcony. He said the girl’s mother was “in the worst pain.”
The Kenyan Red Cross said it had treated at least 93 people injured in the violence.
Most of Kenya is calm
Violence in Kenya could have ripple effects far beyond the nation of 47 million people.
As the largest economy in East Africa, Kenya is a crucial trade route to the continent and provides an important buffer of stability in a region that includes the fledgling Somali government and the politically tense Sudan and South Sudan.
Beyond the deadly incidents in Kisumu and Nairobi, most of the nation remained relatively calm Saturday.