Millions of Belarussians are preparing to vote in presidential elections on Sunday amid claims by the opposition that the ballot has been rigged.
Opponents of President Alexander Lukashenko have called for a mass rally after polls close at 2000 (1800GMT), to protest against electoral fraud.
Mr Lukashenko has warned he will crush any attempt to stage a popular revolt.
His administration has been widely criticised in the West for its authoritarian rule.
Prior to the vote, scores of opposition supporters were detained and a number of foreign election monitors barred.
Chief opposition candidate Alexander Milinkevich predicted that fraud would return President Lukashenko to office with 70% of the vote.
Mr Lukashenko has warned he will not tolerate any attempt at a "coup" and has vowed to "break the neck" of anyone who tries to seize power.
Polls: 0600GMT to 1800GMT
6,627 polling stations (41 abroad)
More than 7m eligible voters
Election valid if over 50% turnout
Lukashenko seeking third five-year term
Police have said that protesters "trying to destabilise the situation" will be treated as terrorists and could face the death penalty.
A spokesman for the human rights organisation, the Helsinki Committee, told the BBC the Belarussian government's threats to opposition groups could trigger violence.
"We've had similar statements during previous elections... This one is stronger and maybe it's a sign that we should expect violence during Sunday evening.
"They are just trying to intimidate people, to make them afraid of participating in protest actions," Dzmitry Markusheuski said.
'No to lies'
Officials in the ex-Soviet republic have accused foreign countries of backing opposition candidates in the election and a number of foreign monitors and journalists have been denied entry to watch the vote.
Mr Lukashenko, a close ally of Russia, is running for his third term in office and points to economic growth and social stability as his main achievements.
Opinion polls were rare during the election campaign but those taken suggested Mr Lukashenko would win the first round even in a fair vote.
Mr Milinkevich has said he is not planning a revolution and he accused the authorities of blatant election propaganda.
"I had expected the campaign to be unfair and dishonest but I had not expected it to be so outrageous," he said.
"We will go peacefully, without violence. We will go onto the square and say: 'No to falsification, no to lies'. Do we have that right? We do," he said.
Mr Lukashenko would, he said, win a landslide but "half of Belarus" would be laughing at the result.
Alexander Kozulin, another opposition candidate, told a rally in Minsk on Friday that it was "time for Belarus to awaken and shake off its fascist dictatorship".
He stressed, however, that protests should be non-violent.
'No conflicts here'
In a final televised election address on Friday, President Lukashenko warned foreign monitors not to "exceed their remit".
"Observe, draw conclusions, but do not decide for us... what path the Belarussian people should take," he said.
Mr Lukashenko defended his record in office, saying the economy was strong and the republic had avoided "social, political, racial and religious conflicts".
Arguing that the presence of four rival candidates was proof of democracy, he warned that political violence would not be tolerated.
"I guarantee that there won't be a coup d'etat in this country," he said.
Large-scale street protests led to the fall of leaders in other ex-Soviet republics such as Ukraine and Georgia.
Both the European Union and the United States have expressed concern at the Belarussian government's tough stance on political protest.