LUSAKA (Reuters) - Zambia's main opposition leader filed a court petition on Friday challenging President Edgar Lungu's re-election at the helm of Africa's second-largest copper producer, saying the vote was rigged.

The southern African nation's economy is buckling under weakened commodity prices, mine closures, rising unemployment, power shortages and soaring food prices that Lungu's rival, Hakainde Hichilema, blames on the current administration.

"We have filed the petition. We are asking for the nullification of the election," Gilbert Phiri, a lawyer for Hichilema's United Party for National Development (UPND), told reporters.

Lungu and the electoral commission, an independent state agency set up by the constitution, who are among the respondents named in the petition, have rejected Hichilema's accusation that fraud discredited the Aug. 11 vote.

Lungu's inauguration has been postponed because a rule introduced in January says the winner of a presidential vote cannot be sworn in if the vote is contested in a court, which will have two weeks to decide on such a petition.

In the petition, Hichilema, an economist and businessman and an old rival of Lungu, says that the president did not win the election legally as he failed to score more than the number of votes required to be declared the winner.

To win, a presidential candidate must garner 50 percent of the valid votes cast plus at least one additional vote.

"The first respondent did not receive more than 50 percent of the total votes cast. The voter register was not credible and its non-availability before the elections compromised the transparency of the electoral process," the petition says.

In the petition, Hichilema demanded a recount, verification and scrutiny of the votes cast at the general election to ascertain the real winner.

He also says in the petition that the number of ballots cast in favor of Lungu was inflated by fake, pre-marked ballots, and asked that the court nullify Lungu's victory.

Lungu won 50.35 percent of the vote against 47.63 percent for Hichilema, according to the electoral commission.

(Reporting by Chris Mfula; writing by James Macharia; editing by Mark Heinrich)