Raila Odinga will be the flag-bearer for Kenya's oppositional coalition, the National Super Alliance (Nasa), in the August 8 election.
Mr Odinga was unveiled at Uhuru Park in Nairobi Thursday afternoon at a ceremony attended by a multitude.
He will be taking on the incumbent Uhuru Kenyatta and other fringe candidates.
President Kenyatta will be seeking a second and final five-year term.
President Kenyatta and his deputy William Ruto are the leaders of the Jubilee Party (JP), formed following the merger of several formations.
If elected, the Nasa coalition will change the structure of the Kenya government, in an apparent bid to accommodate its principal architects and maintain the opposition unity.