Zambia's incumbent president is closing on his main rival as the counting of ballots from Thursday's poll continues.
The counting process has been criticised for its slowness
President Levy Mwanawasa of the Movement for Multiparty Democracy has done well in rural areas, but his rival Michael Sata still maintains a lead.
Mr Sata of the Patriotic Front has warned of "ghastly consequences" if electoral fraud is detected.
The electoral commission's handling of the poll was praised by international observers as efficient and transparent.
But the commission has also come under increasing criticism for the slow pace of result announcements.
Its chairwoman, Ireen Mambilima, said Mr Mwanawasa had clinched landslide wins in central and eastern Zambia, while Mr Sata continued to score highly in the mineral-rich Copperbelt province.
Zambia has a total of four million registered voters, and officials said turnout for Thursday's election was high.
After the counting of about one million votes, Mr Sata had won a share of about 39%, with Mr Mwanawasa on 32%.
Four million Zambians were registered to vote
Hakainde Hichilema of the United Democratic Alliance, a wealthy businessman popular with the middle class, was on 27%.
In the parallel parliamentary poll, the electoral commission said Mr Sata's party had won 26 out of the 49 constituencies that had already reported. Zambia has a total of 150 constituencies.
Mr Mwanawasa's campaign for a second and final five-year term was based on his strong economic record.
He and his backers have boasted about steady economic growth and success in getting most of the country's foreign debt written off.
But opposition supporters say ordinary people have yet to feel the affects of the economic reforms - the revival in the crucial copper sector is slow, unemployment is high and there is concern over health and education standards.
Mr Sata has vowed to secure a more equitable distribution of wealth.