Zambian President Edgar Lungu maintained a slim lead over his main rival on Sunday with nearly half of votes counted as the opposition called for greater urgency in releasing results amid concerns about rigging.
Lungu faces a stiff challenge from Hakainde Hichilema of the United Party for National Development (UPND) who accuses him of failing to steer the economy out of its slump after Africa's second-largest copper producer was hit by weak commodity prices.
Lungu led with 669,960 votes against Hichilema's 644,132 after 69 of the country's 156 constituencies in Thursday's vote had been collated, the Electoral Commission of Zambia (ECZ) told reporters on Sunday.
Lungu's Patriotic Front (PF) party and the UPND have both said they believe they have won the election.
The winning presidential candidate must receive more than 50 percent of the valid votes cast or the two leading contenders will go into a second round of voting.
The UPND has raised concerns that the slow pace of vote counting is due to some ECZ officials trying to manipulate the result in favor of Lungu, a claim the ECZ denies.
"They have taken long to release the results. In the normal set up they should have been released by Friday," Hichilema told reporters on Sunday.
The ECZ had hoped to have final results from the elections - in which Zambians also chose members of parliament, mayors and local councilors and decided on proposed constitutional changes - by early Sunday. Results were now expected later, officials said, without giving a time frame.