Stepan Sukhorenko, the head of the security service, still known by its Soviet initials KGB, told a news conference that protests would be treated as "terrorism".
"We are obliged to announce that under cover of elections, a violent attempt to seize power is being planned in the country," he said.
"The actions of those who take the risk of going into the streets to attempt to destabilise the situation will be viewed as terrorism."
Sukhorenko was speaking three days before the vote in which Alexander Lukashenko hopes to be re-elected.
His announcement was similar to previous statements that the security services had uncovered Western-backed plots to destabilise the former Soviet state of 10 million, accused in the West of crushing human rights.
A man in Minsk votes early
Earlier this month, he went on television to announce that activists had been plotting explosions in the run-up to Sunday's poll.
But the timing of the latest statement added drama to the election, in which Lukashenko is heavily favoured to defeat three other candidates - two from the liberal and nationalist opposition.
Western countries have threatened to toughen sanctions against Belarus if, as expected, independent observers declare the election neither fair nor free.
Alexander Milinkevich, the main opposition candidate, has urged supporters to mass at polling stations to keep the count honest. Several key opposition figures have been rounded up in recent weeks and fined or sentenced to short prison terms.
"The key moment will be detonation of several explosions"
Lukashenko, in power since 1994, has said he will tolerate no upheaval like the mass protests that helped bring liberals to power in former Soviet Georgia and Ukraine.
Sukhorenko showed reporters footage of an unidentified man saying he had undergone military training at a camp in Georgia.
"Brigades of volunteers are being formed in neighbouring states," Sukhorenko said. "The key moment will be detonation of several explosions."