The main opposition candidate in Belarus, Alexander Milinkevich, has called for a re-run of the presidential election within hours of polls closing.
Several thousand Milinkevich rallied in central Minsk
He made the demand as several thousand protesters filled a central square in Minsk, amid claims of vote rigging by President Alexander Lukashenko.
Preliminary official results show President Lukashenko won re-election with 82.6% of the vote.
The president has warned he will crush any attempt to stage a popular revolt.
Central Election Commission head Lidia Yermoshina said Mr Milinkevich had obtained 6% of the vote.
Turnout was 92.6%, the commission said.
About 10,000 people braved the freezing temperatures and threat of government reprisals to show their support for the opposition and gather in Oktyabrskaya Square.
They shouted slogans, waved national flags banned under Mr Lukashenko, and booed pictures of the president being shown on a large television screen.
6,627 polling stations (41 abroad)
More than seven million eligible voters
Election valid if over 50% turnout
Lukashenko seeking third five-year term
But the BBC's Emma Simpson in Minsk, says there is no evidence of any popular uprising such as the large-scale street protests which led to the fall of leaders in other ex-Soviet republics such as Ukraine and Georgia.
Mr Milinkevich told the crowd that claims his rival Mr Lukashenko had won a landslide victory, and thus a third term in office, were a lie.
"We demand new, honest elections. This was a complete farce," he said.
The police presence at the demonstration has so far has been low key, our correspondent says, although buses of security service men are not far away.
Mr Lukashenko, whose administration has been widely accused in the West of imposing authoritarian rule, 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.
The official results give Lukashenko an easy victory
Police have said that protesters "trying to destabilise the situation" will be treated as terrorists and could face the death penalty.
Both the European Union and the United States have expressed concern at the Belarussian government's tough stance on political protest.
A spokesman for the Helsinki Committee, a human rights organisation, 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," Dzmitry Markusheuski said.
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 were denied entry to watch the vote.
For his part, Mr Milinkevich has repeatedly called for any protests to be peaceful.
Milinkevich says he is not trying to foment a revolution
"I want to say again that I, as a presidential candidate, or representatives of my headquarters, have never called for a revolution. We are categorically against any revolutions," he said prior to the rally.
Mr Milinkevich earlier said that he thought he could win the election if votes were counted honestly.
But our correspondent says that despite his authoritarian regime Mr Lukashenko has genuine popular support.
Opinion polls were rare during the election campaign, but those taken suggested Mr Lukashenko would win the first round even in a fair vote.