Opposition leader Kumba Yala made the statement to reporters Thursday.
Former prime minister Carlos Gomes Junior won last Sunday's election with 49 percent of the vote, but electoral laws mandate a runoff if no candidate wins a majority.
Yala is one of five candidates who want the first-round election annulled because of alleged irregularities.
International election observers have said Sunday's poll appeared to be free and fair.
The fraud claims have raised fears that trouble is looming in the coup-prone country, where former military intelligence chief Samba Diallo was killed hours after the polls closed.
The candidates were vying to replace the late president Malam Bacai Sanha, who died in January after a long illness.
Since winning independence from Portugal in 1974, Guinea-Bissau has struggled through a dictatorship, three coups and the 2009 assassination of a president.
The country has also become a transit point for cocaine smugglers.
Some information for this report was provided by AP, AFP and Reuters.