Zambia's ruling party candidate in presidential elections has taken the lead with more than half of the votes counted, as polling continue in some parts of the country.

With 90 out of 150 constituences counted on Thursday, Edgar Lungu of the Patriotic Front party was ahead with 50.75 percent of votes, the Electoral Commission of Zambia (ECZ) said.

He was closely followed by opposition leader Hakainde Hichilema of the United Party for National Development with 45.14 percent of the vote.

The election was held on Tuesday to replace President Michael Sata, who died in October after a long, undisclosed illness. The winner will serve out Sata's term until elections next year.

Turnout was around 34 percent as heavy rains disrupted voting across much of the landlocked country. Observers said the election was conducted in a fair manner.

The final result was due to be announced on Friday but could be delayed.

Voting was continuing at about 12 polling stations across the country after heavy rain disrupted Tuesday's vote, forcing an extension.

ECZ chairperson Ireen Mambilima said the country's air force was helping deliver election material from polling stations that could not be reached after the heavy rains damaged some roads.

"I think if the Zambia Air Force will meet their target we are hopeful that we can finish tomorrow," she said on Thursday.

Cheaper food

Lungu, a former lawyer, has used his campaign to tap into the grassroots support of his predecessor Sata, promising voters cheaper food and fuel.

Hichilema, one of Zambia's wealthiest businessmen, has said if he wins he will draw on his experience in the private sector to attract foreign investment and diversify the economy, in a country where copper accounts for 70 percent of export earnings.

Zambia has averaged six to seven percent growth as the mining sector boomed but that slowed to 5.5 percent last year, the International Monetary Fund said, and could ease further with the price of copper reaching a six-year low this month.

With another election due late next year when Sata's term would have ended had he not died in office, the winner will have little time to turn around a stuttering economy in one of Africa's most promising frontier markets.