At least 500 opposition supporters marched in Minsk on Sunday
Parliamentary polls in Belarus - in which the opposition failed to win a seat - "fell short" of international standards, European monitors say.
However, the Organisation for Security and Co-operation in Europe said there were "minor improvements" during Sunday's elections.
All 110 seats went to pro-government candidates, electoral officials said.
The opposition says the vote was not legitimate, but President Alexander Lukashenko says it was free and fair.
OSCE monitors said in a statement that the vote was "generally well conducted, but the process deteriorated considerably during the vote count".
They said they were denied full access to more than one-third of polling stations for the count and found "several cases of deliberate falsification of results" at other locations.
The OSCE deployed some 450 monitors in the former Soviet republic.
Rally in Minsk
Earlier, opposition parties said the result was rigged, urging international monitors not to recognise the outcome.
The US has described Mr Lukashenko as Europe's last dictator
"There was no election in Belarus. It was an electoral farce for the West," Anatoly Lebedko, leader of the opposition United Civil Party, was quoted as saying by the Associated Press news agency.
At least 500 opposition supporters held a peaceful protest rally in the capital Minsk after the polls closed late on Sunday.
Elections commission chief Lidia Yermoshina said the conduct of the vote was "in accordance with the law".
"Not a single opposition candidate was elected," she said, adding that turnout was 75%.
President Lukashenko - who has been in power since 1994, in what critics have called Europe's last dictatorship - has said he expects the polls to lead to better relations between Belarus and the West.
In a bid to appease his critics, the president ordered some opponents to be freed from prison and allowed about 70 opposition candidates to stand.
Mr Lukashenko has been trying to re-build ties with the West after a cooling in his relations with Russia, his long-time ally.
European and US diplomats had suggested that sanctions against Belarussian companies could be lifted if the election was conducted well.
All 110 seats of the House of Representatives in Belarus are currently occupied by politicians who back Mr Lukashenko.