Lusaka - Rioting broke out in Zambia's capital overnight, after it emerged that acting president Rupiah Banda had overturned opposition leader Michael Sata's lead in Thursday's presidential by-election with only a few results left to count, reports in South Africa said.

With just two constituencies out of 150 in the southern African country left to be announced, Banda, 71, the candidate of the ruling Movement for Multi-Party Democracy, was poised to become president for the next three years, replacing ex-leader Levy Mwanawasa, who died in August.

Sata, also 71, has accused electoral authorities of rigging in Banda's favour and gone to court to demand that the count be stopped.

"We will not recognize Banda unless a court tells us why we should," a spokesman for Sata's Patriotic Front, Given Lubinda, told South Africa's SAfm radio, saying the party would demand a recount.

Sata has complained of numerous alleged irregularities during the vote. Among other things, he claims voting continued in some places while the first results were being announced, SAfm reported.

African observers gave the election a clean bill of health.

SAfm reported that sporadic rioting took place overnight in some poor neighbourhoods of Lusaka after news of Sata's electoral setback trickled through. Lusaka is a Sata stronghold. Banda is more popular in rural areas.

Sata, who is on his third attempt to become president, had looked comfortably ahead at one point, while the rural votes were still being counted.

The Zambian army has been on alert since earlier this week , amid fears of violence in the event of a disputed outcome.

In 2006, Sata's initial refusal to accept his defeat at the hands of Mwanawasa sparked days of rioting.

Zambia is Africa's largest copper producer but 65 per cent of the population live on less than a dollar a day.

The global financial crisis has loomed large over the election, as fears of a worldwide recession drag down prices for the commodity.

Banda has promised to continue Mwanawasa's legacy of prudent economic management, while cutting taxes on food and fuel.

Sata is promising to cut taxes and give Zambians a stake in foreign-owned mines. (dpa)