Samura Kamara, head of the All People's Congress (APC) party, said he wanted the chief electoral commissioner and all regional commissioners of the national election body to be replaced by "an independent internationally accredited team."
"We do not have a credible final voters register," Kamara told Reuters late on Wednesday, following an address to the nation.
"The production of blurred and substandard voter identity cards, the repeated failure to meet deadlines regarding the submission of credible voter registration data, and the subsequent release of highly questionable data, have raised serious doubts about the Commission's commitment to conducting free and fair elections," he said.
The West African country will go to the polls on June 24 to decide whether President Julius Maada Bio will get a second term. Kamara was the runner-up to Bio in a close race in 2018.
Sierra Leone's chief electoral commissioner Mohamed Kenewui Konneh rejected Kamara's accusations and said none of his team would resign.
"We have addressed all these issues – in person and in writing," he said. "The ballot papers will be here over the weekend and we are going ahead with the elections."
Kamara said that if the officials did not resign within 72 hours, the APC party would meet and "decide the next steps."
On Tuesday, dozens of young people were arrested following opposition protests in several cities calling for the electoral commissioner to resign, local media reported.