"In recent days we have been getting more and more reports of vote buying," interior minister Ivan Demerdzhiev told reporters at a briefing during a graduation ceremony for Interior Ministry Academy cadets.
"We have signals that an attempt is being made to generate votes, especially in support of one of the pairs," the minister said.
Demerdzhiev said the first round went calmly, with no vote buying. He called on everyone taking part in the election to carry on in the same way.
The Interior Ministry will step in wherever there are reports of vote buying, he said.