Hackers attacked the website of Zambia's Election Commission on Thursday, posting a string of false results showing opposition leader Michael Sata in the lead and causing delays to the release of the official tally.
The process appears to be slow because the results need to be verified before release, Commission spokesman Cris Akufuna said. Election officials are running double and triple checks with regional counting centers.
According to the latest confirmed tally from 33 of 150 constituencies in Africa's biggest copper producer, Sata had 265,843 votes against 192,936 for incumbent President Rupiah Banda of the Movement for Multi-party Democracy (MMD).
Banda is expected to perform the stronger in the countryside, which is likely to report votes more slowly than Sata's strongholds in the capital, Lusaka, and the Copper Belt, the country's economic heartland.
It is therefore too early to say whether Sata is on the verge of an historic transfer of power in the former British colony, removing the MMD from the presidency for the first time since the end of one-party rule in 1991.
Results are still being tallied in some constituency centers, and we expect to make significant progress by the end of the day, Akufuna said.
The election was held on Tuesday and it may now take until the weekend for a complete result to be known.
Sata, known as King Cobra for his vicious tongue, has toned down his rhetoric against foreign mining firms, most notably those from China. But a victory for the 74-year-old would still cloud the investment outlook for what has been one of frontier Africa's most attractive prospects.
He lost to Banda, also 74, by just 35,000 votes, or 2 percent of the electorate, in a 2008 presidential run-off triggered by the death in office of Levy Mwanawasa.
(Writing by Ed Cropley; Editing by Jon Herskovitz)