Lusaka - Allegations by Zambia's opposition leader Michael Sata of vote rigging cast a pall over Thursday's election to choose a successor to deceased president Levy Mwanawasa and stoked fears of violence in Africa's largest copper producer.

The mood on voting day itself was calm as Zambians made their way from dawn to over 6,000 polling stations nationwide to choose a successor to Mwanawasa, who died of a stroke in August.

The election is a two-horse race between ruling Movement for Multi-Party Democracy (MMD) candidate, acting president Rupiah Banda, 71, and opposition Patriotic Front leader Sata, also 71. Two other candidates are also running.

After casting his vote in the capital Lusaka, the populist Sata accused the police and army of working with the Zambia Electoral Commission (ZEC) to rig the elections in Banda's favour. The ZEC has persistently rejected allegations by Sata of plans to rig the vote.

Sata also said threats by army commander Isaac Chisuzi this week to deal harshly with people inciting violence were aimed at intimidating voters.

Early voter turnout was sluggish in much of the country but the pace picked up during the day.

An outdated voters roll of only 3.9 million voters for a population of around 11.7 million people was blamed for low voting levels in some areas.

An electoral officer in the Copperbelt town of Kitwe said some voters there were turned away because they did not have the proper ID or their name was not on the voters roll. The number of such disenfranchised voters was not known.

Police were deployed at all polling stations amid fears of a repeat of the violence that gripped the last presidential election in 2006. Sata's initial refusal that year to accept defeat at the hands of Mwanawasa sparked days of rioting that ended only when the army was deployed.

Buoyed by two pre-election opinion polls showing him winning, Sata has threatened to again reject the outcome of the election if he suspects vote rigging.

Nicknamed King Cobra for his attacking political style, Banda has attracted large crowds at his rallies with his promises to cut taxes and give Zambians a greater stake in mining companies and parastatals.

At a school in the low-income Garden Compound district of Lusaka, where Sata is popular, a timber trader who had just voted for Sata, said the country needed change after almost 17 years under the MMD.

"It is like Zambia has been turned into a one-party state and democracy no longer prevails," Chisomo Tembo told Deutsche Presse- Agentur dpa.

Banda has campaigned on a theme of continuity with Mwanawasa's prudent economic policies, which won over foreign investors, particularly Chinese investors in mining, and Western donors who forgave Zambia millions of dollars in debt.

The economy has weighed heavily on the election. Copper prices are plummeting as slower global growth depresses demand for commodities.

While posting strong growth in recent years, Zambia is still ranked among the poorest countries in the world, where about 65 per cent of its people live on less than a dollar a day.

ECZ spokesman Chris Akufuna said counting would begin directly after voting ended at 6 pm (1600 GMT). The ECZ has said the first results could be in by Friday. (dpa)