"The height of the pile of ivory before us marks the strength of our resolve," Kenyatta said, before setting fire to the pyres.
"No one, and I repeat no one, has any business in trading in ivory, for this trade means death of our elephants and death of our natural heritage."
Eleven giant pyres of tusks, and another of rhino horns, are arranged in a semi-circle now expected to burn for days in Nairobi's national park.
President Ali Bongo from Gabon, who lit one of the pyres, spoke of the "massacre" of forest elephants in central Africa, and said he backed moves to close all sale of ivory.
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"Unless we take action now, we risk losing this magnificent animal," Bongo said at the ceremony, telling poachers he was "going to put you out of business, so the best thing you can do is to go into retirement now."
The pyres contain some 16,000 tusks and pieces of ivory.
Kenya has a long history of ivory burnings, spearheading a wider movement of public demonstrations across the world, but nothing on this scale before.
On the black market, such a quantity of ivory could sell for over USD 100 million, and the rhino horn could raise as much as USD 80 million.
But, Kenyatta dismissed those who put cash value on the ivory.
"For us, ivory is worthless unless it is on our elephants," Kenyatta said.