After polling stations closed on Sunday, about 1,000 opposition activists marched through October Square in the capital Minsk claiming there had been widespread electoral fraud.
Protesters held banners declaring "No to Farce," "Dictatorship Should Go to the Dustbin of History," and "No to Russian Military Bases".
They also waved flags of the European Union and the old red-white-red Belarus flag thrown out by Alexander Lukashenko, the Belarussian president, in 1996.
Maya Stachevskaya, one of the protesters, said: "Europe should not recognise these elections. Lukashenko does not organise free elections, he just names winners."
Al Jazeera's Jonah Hull, reporting from the rally, said: "These people were clearly unimpressed with the election ... they want a total overhaul of the Lukashenko system and what is often described as Europe's 'last outpost of tyranny'.
"Ordinarily these kind of gatherings would be swiftly shut down by riot police. That's an optimistic sign for outside observers."
Alexander Milinkevich, an opposition leader, told the rally that the vote had not been fair or democratic.
"We still have no democratic polls," he said.
"One can speak about some cosmetic changes, but these elections cannot be described as matching OSCE (Organisation for Security and Co-operation in Europe)principles. I think the OSCE will say so."
Lukashenko criticised opposition groups on Sunday for taking "outside" funding.
"A real, constructive opposition is always needed ... but not an opposition fed and financed 100 percent from outside," he said.
Election observers from the OSCE were due to report at 3pm (12:00 GMT) whether they had found the poll in Belarus to be free and fair.
Earlier, a senior Western election observer in Belarus has said that authorities had made "real efforts" to increase fairness, citing the increased number of opposition candidates and the greater time they have been given on television.
'Europe's last dictator'
Sergei Kalyakin, leader of the opposition Communist Party, told reporters on Sunday that his election monitors had failed to record any major wrongdoings during the vote.
|The opposition has criticised the
system of advance voting [AFP]
But he criticised the process of advance voting, which began on September 23, saying it gave the government an opportunity to cheat because ballot boxes were not monitored as closely as on election day.
More than 25 per cent of the electorate cast their ballots between Tuesday and Saturday, the election commission said. The total turnout after Sunday's voting was about 75 per cent.
No election in the former Soviet nation has been endorsed by Western organisation since the mid-1990s, but the United States and European Union have signalled that relations could be improved if there was tangible democratic progress.
Lukashenko, once referred to by Washington as "Europe's last dictator", has freed political prisoners and eased restrictions on the opposition, which was shut out of the previous parliament.
"If the election goes smoothly, the West will recognise Belarus," Lukashenko, who is currently banned from travelling to the US and EU countries over accusations he rigged his re-election in 2006, said after voting on Sunday.
Alexander Kozulin, a leading opposition member who was among prisoners released last month, said that the result showed that Lukashenko had "closed the door that the West was trying to open for him".