KGB Chairman Stepan Sukhorenko said the opposition had intended to gather several thousands of people in the national capital Minsk after the announcement of presidential election results, and to blow up an explosive device during the rally.
Following this, the opposition planned to declare the election results false, seize administrative buildings and railway stations, and block railroads, the security chief said.
Sukhorenko said that representatives of some Belarusian NGOs, as well as militants from Ukraine, Georgia, and former Yugoslavia could have been brought into the coup.
"The unregistered [non-governmental] organizations have gone underground, and are acting with a strong level of professionalism, as most of their members have been trained abroad."
Around 100 mobile phones with Lithuanian SIM-cards and several tens of thousand dollars had been seized from one of these organizations, Partnership.
He said the organization was financed by a regional branch of the National Democratic Institute, and U.S. citizen David Hamilton. The KGB chief said the United States had allegedly allocated $12 million to support these activities in 2006.
The official said the plotters would be unlikely to implement their plans now that they have been made public.
"However, we will monitor the situation, and if they risk doing this, we will find the explosive devices."
"We know both the organizers and perpetrators," he said, adding that they would be detained if they attempted to carry out their plan.
Alexander Milinkevich, the democratic opposition presidential candidate, said earlier that it would be announced that incumbent President Alexander Lukashenko had gained at least 75% of the vote in the elections.
He said Belarusians wanted change, and wanted to live in a different country.
"If we get rid of the information vacuum, the existing Belarusian regime will fall," he said.
He also said the opposition would not hold any rallies, even if they lose the elections.
According to Milinkevich, the situation in Belarus was alarming, as "the elections can already be considered non-democratic."
"The elections are being held under total falsification and persecution of the opposition. We are taking part in this political campaign in order to destroy the fear that reigns in our society."
The incumbent president, Alexander Lukashenko, dubbed "Europe's last dictator" by the Western media, has ruled Belarus since 1994.