A Zambian woman casts her vote at a school in Lusaka, Zambia, on Thursday. (Themba Hadebe/Associated Press)

The main opposition candidate in Zambia's presidential election accused the ruling party of rigging the vote as he cast his ballot Thursday, an indication he will contest election results.

Michael Sata's party has raised questions about the number of ballot papers printed for the election being held on Thursday, saying extra ones could be used fraudulently.

Sata criticized army commander Gen. Isaac Chisuzi on Thursday for saying on national television the day before that his troops were prepared to keep the peace if the announced election results spark violence.

"When the army commander can intimidate voters like he did last night, it means they have already rigged the elections," Sata said after voting in the Zambian capital of Lusaka.

The governing party candidate, acting President Rupiah Banda, has promised a free and fair election, which was called to replace the late president Levy Mwanawasa, who died in office in August after suffering a stroke.

The vote could be close, but independent observers predicted Banda would win and complete the three years remaining Mwanawasa's second term in office.

Turnout was heavy, but lines moved smoothly and quickly throughout the morning and into the early afternoon.

The country's largest independent election monitoring group, meanwhile, said that while the election was generally peaceful, there were some instances of voting irregularities.

The Foundation for Democratic Process said on Thursday there were some issues regarding proper verification and identification of voters.

But another independent group said they were satisfied with the process.

"We are happy with the buildup of the election so far," John Kunene, head of the election observer team sent by the Southern African Development Community, told reporters as he toured polling stations Thursday. "It has been peaceful."

The European Union also sent observers.

Earlier, delays of about 30 minutes in opening several Lusaka stations were blamed on the late arrival of party representatives who are legally required to monitor the vote. Capital residents stood patiently chatting in long lines at the polls.

Albert Hachinene, a government worker waiting to vote in Lusaka, said campaigning and debating was over.

"The actual decision will be made here today," he said. "It's good we are voting so that we can choose a leader of our choice."

Economy top priority for parties

At their final rallies Wednesday, the main contenders called economic progress their top goal.

Banda vowed to continue the popular Mwanawasa's free-market legacy, while Sata promised relief from high taxes. Sata's populist rhetoric may be resonating, as prices for copper – the mainstay of Zambia's economy – plummet, inflation rises and fears of interest rate hikes grow.

Both agreed that Zambia needs even more of the Chinese investment first welcomed by Mwanawasa. The late president won praise for fighting corruption and modernizing Zambia's economy but admitted he had failed to lift the nation out of crushing poverty.

Mwanawasa's stewardship saw foreign investment increase to an estimated $4 billion in 2008 from $71.7 million in 2001, with $1 billion of the increase coming from China.

Nearly 4 million of Zambia's 13 million people were registered to vote. Turnout in 2006 was more than 70 per cent.

Final results may take up to a week because of poor communications.

With files from Reuters