Zambian President Edgar Lungu narrowly won re-election in a vote.
Lungu, who narrowly beat Hichilema in a vote last year to replace late president Michael Sata, won 50.35 percent of the vote against 47.63 for his opponent.
Hakainde Hichilema’s United Party for National Development (UPND) said it would appeal the result at the Constitutional Court, accusing election officials of fraud during the count.
The ruling party and the electoral commission have rejected the UPND’s charge.
The election dispute, which could blight Zambia’s reputation as one of the most stable democracies in Africa, could damage negotiations with the IMF.