Opposition leader Hakainde Hichilema alleges some electoral officials are colluding in favour of the governing party.
With the main opposition party in the southern African country of Zambia accusing the government of foul play, President Edgar Lungu on Saturday took the lead in early counting in the country's presidential election.
Hakainde Hichilema, leader of the opposition United Party National Development (UPND), has alleged that some electoral officials were colluding in favour of the governing party.
Hichilema has also accused Lungu of failing to steer the economy out of its slump after Zambia – Africa's second-largest copper producer – was hit by weak commodity prices.
President Lungu led with 207,547 votes against Hichilema's 153,633 after 22 of the country's 156 constituencies in Thursday's voting had been collated, the Electoral Commission of Zambia (ECZ) told a news conference also attended by political parties.
Early results announced on Friday from only eight constituencies had put Hichilema ahead.
The commission has rejected UPND charges that some of its officials were working to manipulate results to the advantage of Lungu's Patriotic Front.
It said police were still investigating a report that an ECZ official had given his identity card on Friday to a man who could then enter the commission's computer room and tamper with the results.
The UPND renewed its calls for the commission to remove some officials from the election process to preserve its credibility.
Slow vote count
The ECZ has also defended the relatively slow pace in announcing election results, saying audits were taking longer than expected mainly due to a large voter turnout.
As of Saturday's count, the turnout was at 56.32 percent, far above the 32 percent recorded early last year when Lungu narrowly won an election to fill the vacancy left by the death of then president Michael Sata.
If no candidate fails to win more than 50 percent this time, Zambia will have to hold a second round of elections.
Campaigning for this week's vote centred on the economy, after months of rising unemployment, mine closures, power shortages and soaring food prices.
Supporters of the two main parties clashed in what is normally considered to be one of the continent's most stable democracies.
With emotions running high as parties await the election results, the ECZ would need to clearly demonstrate it was acting to resolve complaints to retain the confidence of the electorate, political analyst Lee Habasonda of the University of Zambia said.
Hichilema says Lugu has mismanaged the economy, but the president – whose government has been negotiating a financial support package with the International Monetary Fund – blames weak growth on plunging commodity prices.
The ECZ said final results from the elections, in which Zambians also chose members of parliament, mayors, local councillors, and proposed constitutional changes, would not be in by late Saturday or early Sunday as initially anticipated.