Victoire Ingabire, who is under house arrest and faces charges of terrorism and negating the 1994 genocide, said the world was turning a blind eye to the crisis in Rwanda.
"While some members of the international community and even a handful set of militant diplomats fool the world that the ongoing sham election has some sort of credibility, the country is getting closer to the brink of chaos," she said.
"The world is failing Rwanda once again, and the democracy ideal is thwarted. A new long crisis is looming," said Ingabire, whose United Democratic Forces was denied official registration, disqualifying her from the election.
Rwandan President Paul Kagame has been leading a high-octane and expensive campaign over the past two weeks and is almost assured of re-election on August 9, facing the challenge of only three politicians with ties to his regime.
Opponents and rights have criticised Kagame for tightening the noose on any dissident voices ahead of the election but the authorities have dismissed the accusations as unfounded and the result of a smear campaign.