The commission said Kenyatta won Tuesday's election with 54 percent of the vote.
The opposition rejected the results even before they were declared, calling the process a "charade".
Before the results were read, one of the leaders of the opposition Nasa alliance, Musalia Mudavadi, said concerns about the poll had not been adequately addressed.
President Uhuru Kenyatta watched election results live on television on Friday, minutes before going on stage to re-confirm his second term in office.
After the announcement, Mr Kenyatta called for unity, telling opposition supporters: "I reach out to you. To our brothers, our worthy competitors, we are not enemies, we are all citizens of the same republic".
"I extend a hand of friendship, I extend a hand of cooperation, I extend a hand of partnership, knowing full well that this country needs us, all together".
"There is not a single Kenyan anywhere who wants to continue to see violence, looting and demonstrations that end up destroying properties", he said, striking the same conciliatory tone he used when accepting victory on Friday.
Following the announcement of results, hundreds of police in anti-riot gear were in the streets of the capital, Nairobi, amid fears of further protests by opposition supporters.
Top Odinga lieutenant Johnson Muthama told Reuters police proceeded to pack corpses into body bags before dumping them.
In 2007 an opposition coalition led by Odinga disputed the result after incumbent Mwai Kibaki was announced the victor by the electoral commission.
Protests erupted in Odinga's strongholds in western Kisumu county and poor areas of Nairobi nearly immediately after the election results were declared Friday, with gunshots ringing out and fires in the streets.
In 2013 he said the election was rigged and took his case to the Supreme Court where he lost.