Zambia's main opposition leader has filed a petition at the constitutional court challenging last week's re-election of President Edgar Lungu.
Hakainde Hichilema of the United Party for National Development, or UPND, says the vote was rigged and Lungu did not receive the 50 percent required to avoid a run-off.
Zambia's electoral commission said Lungu, of the ruling Patriotic Front, won 50.35 of the vote against 47.63 for Hichilema.
The court challenge will delay Lungu's scheduled inauguration because of a law that says a president cannot be sworn into office if the election is contested in court. The constitutional court has 14 days to hear the petition.
"What we are asking for is the nullification of the election," said Gilbert Phiri, a lawyer for Hichilema.
Zambia's electoral commission has insisted the August 11 vote was free and fair.
The election campaign was marked by weeks of clashes between supporters of the rival parties, which saw at least three people killed.
Lungu has served as president since January 2015, following the death of President Michael Sata. He defeated Hichilema in a snap election called last year, winning by just 28,000 votes.
The political tension in Zambia is accompanied by a struggling economy, largely because of a fall in the price of copper, the country's main export. Zambia is in talks with the International Monetary Fund for financial assistance.