Local authorities said they stormed a Multiprint printing house Saturday night and seized 350,000 illegally printed ballots.
Multiprint, the company commissioned to print the official ballots, is owned by a municipal councilor from the center-right GERB party, Sofa News Agency reported.
Officials confirmed Sunday that the seized ballots were ready to be transported as part of the fraud attempt.
Bulgarian opposition leaders said that the GERB party, headed by former Prime Minister Boyko Borisov, was organizing the fraud.
GERB has denied the accusations, Sofa News Agency said.
Exit polls conducted Sunday indicated that the GERB party took a lead of more than 3 percent over its main challenger, the social BSP party.
Support for GERB stood at 30.3 percent, while backing for the Socialists amounted to 27 percent.
The first official results will be available Monday morning, Sofia News Agency said.