Secretary to Zambia's cabinet, Roland Msiska, acknowledged the Aug. 23 date would have to be revised because of the petition.
Zambia's main opposition party will file a court petition against President Edgar Lungu's re-election to the helm of Africa's second-largest copper producer on Thursday, its vice president said on Wednesday.
The United Party for National Development says some electoral officials colluded with the ruling Patriotic Front to rig the vote, handing victory to Lungu over rival and main opposition leader Hakainde Hichilema. Both sides deny the charge.
"We have been robbed of this election, that is a fact and it shall be proved after 14 days," the party's vice president, Geoffrey Mwamba, told a news conference in the southern African nation's capital.
"We have enough evidence to secure us a judgment which has so much being awaited by the Zambian people," Mwamba said.
The inauguration of the president had initially been set for Aug. 23, but a law introduced in January says the winning candidate cannot be sworn in if the vote is contested in the Constitutional Court, which has two weeks to decide on such a petition.
Secretary to Zambia's cabinet, Roland Msiska, acknowledged the Aug. 23 date would have to be revised because of the petition. "The inauguration will depend on how quickly the Constitutional Court disposes of the case," he told Reuters.
The UPND said on Saturday data from its own parallel count showed economist and businessman Hichilema beating Lungu, a lawyer, "with a clear margin" with about 80 percent of votes counted.
The election results released on Monday means Hichilema has now lost five presidential elections.
The election was fought around an economy suffering an economic slump due to depressed commodity prices that has led to mine closures, rising unemployment, power shortages and soaring food prices which Hichilema, blamed on Lungu's mismanagement.
Lungu blames Zambia's economic woes on its over-reliance on copper exports, and says his government has been working to diversify the economy by investing in other sectors like agriculture.