Succes Masra, who came second in Chad's presidential election, announced Sunday he had lodged a request with the Constitutional Council to have the vote annulled.
Masra was acting after dozens of activists from his party were arrested and accused of having forged documents to get illegal access to vote counts.
"With the help of our lawyers, we have today filed a request with the Constitutional Council to reveal the truth at the ballot boxes," said Masra in a Facebook post.
"Our request is for the annulment, pure and simple, of this electoral farce," Sitack Yombatina, vice president of Masra's Transformers Party, told AFP.
"All the evidence is in the USB keys," attached to the request lodged with the Constitutional Council, he added.
They included video footage of voting boxes being stuffed, thefts, threats, "but most of all ballot boxes that were taken away by the soldiers to be counted elsewhere", said Yombatina.
Elections officials on Thursday declared junta leader General Mahamat Idriss Deby Itno the winner with 61.03% of the vote against Masra's 18.53%.
Masra had already claimed victory and warned that Deby's team would try to rig the result.
Dozens of activists from Masra's party have been arrested accused of forgery and using false documents during this week's presidential election, a legal source told AFP on Saturday.
The party has denounced the arrests and the "ridiculous" charges against their activists.
Masra is a former fierce critic of Deby. Although Deby appointed him prime minister four months before the presidential election, Masra ran against him.
The country's opposition, which has been violently repressed and its leading figures barred from standing, had in any case dismissed him as a stooge, allowed to run to give the campaign a "democratic veneer".
Early in the campaign, observers predicted a massive win for Deby, 40, whose top rival was killed earlier this year.
Deby was proclaimed transitional president three years ago by his fellow generals after his father, iron-fisted president Idriss Deby Itno, had been killed by rebels after 30 years in power.
The definitive election results are expected by May 23 at the latest.
AFP