Lukashenko, criticised in the West for crushing human rights during his 12 years in power, says his rivals are Western-funded troublemakers.
His security service, called the KGB as in Soviet times, says protests will be seen as "terrorism".
Lukashenko is all but certain to defeat his challengers. Two of the three, from the liberal opposition, have asked supporters to mass in central Minsk when polls close, as protesters against election fraud did in Ukraine's 2004 "Orange Revolution".
Just more than 7 million people may cast ballots in the ex-Soviet state, which lies between Russia and EU member Poland. Polls are open until 1800 GMT and a result is expected late in the evening.
After a campaign marred by arrests, harassment and terror plot accusations, the opposition have pledged to protest at irregularities suspected as voter fraud. This may set them on a collision course with the authorities, who have banned rallies on election day.
After revolutions that have swept opposition leaders to power in three former Soviet republics after disputed elections, Belarus could be the latest country on Russia's periphery to be convulsed by protests and the threat of a forceful state response.