Kenyatta won a repeat presidential election on Oct. 26 that was boycotted by opposition leader Raila Odinga, who said it would not be free and fair.
The Supreme Court nullified the first presidential election, in August, over irregularities.
The extended election season has divided Kenya, a Western ally in a volatile region, and blunted growth in East Africa's richest economy.
Odinga's supporters, many drawn from poorer parts of the country, feel locked out of power and the patronage it brings.
Political arguments often have ethnic undercurrents, with Odinga's supporters pointing out that three of the country's four presidents have come from one ethnic group, although the country has 44 recognised groups.
But such arguments seemed far from the happy crowds at the celebration, who cheered wildly as Kenyatta was sworn into office and as he received a 21-gun salute.
"I ... do swear ... that I will always truly and diligently serve the people of the Republic of Kenya," Kenyatta said, his hand resting on a Bible.
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