Up to 10,000 protesters marched through the snow-bound capital of Belarus and police beat an opposition leader as a pollster forecast a landslide election victory for Alexander Lukashenko.
Opposition candidate Vladimir Neklyayev was taken to hospital with a head injury after police dispersed a column of his supporters heading to join a protest rally that began in central Minsk's October Square.
Black-clothed security forces fired shots into the air and launched stun grenades after breaking up Neklyayev's group.
In the main rally, thousands of demonstrators poured down Minsk's broad avenues in sub-zero temperatures, chanting 'Out!' and 'Long live Belarus', followed at close quarters by police.
Mr Lukashenko had earlier scoffed at the opposition's plans to protest against an election it says was rigged. He predicted nobody would turn out.
Some demonstrators broke the glass doors to the government building but were restrained by others in the crowd, a Reuters reporter said.
An exit poll conducted by pro-government research group Ecoom gave 56-year-old Lukashenko 79.1% of the vote, which, if confirmed, would hand him a fourth term in power.
He has ruled the former Soviet republic with an iron fist since 1994, its command economy propped up by energy subsidies from chief ally Russia.
But relations with Russia have been on the rocks in recent years, and the moustachioed former state farm director has been courting the West.
The European Union has dangled the prospect of financial aid if today's vote is deemed fair, and is weighing how far to engage with the country of 10 million people that serves as a buffer between Russia and NATO and a transit route for Russian gas heading to Europe.
It will take its cue from Organisation for Security and Cooperation in Europe (OSCE) monitors who report back on Monday.
The OSCE had said earlier in the day that the election already appeared "better" than in 2006