Protesters accused the allies of Alexander Lukashenko, the president, of preventing a fair election.
Al Jazeera's Jonah Hull, reporting from Minsk, 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."
But Hull said that opposition supporters would say that this could all be cosmetic.
"Anyone would be hard pressed to call this election free and fair. The security climate could never permit that.
"What monitors are looking for is improvements in the electoral system ... that they are likely to endorse and could lead to the lifting of sanctions against Belarus."
Earlier the 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.
"The biggest change is that we are actually allowed to participate in the vote count," Anne-Marie Lizin, the international community's special co-ordinator in Belarus, said.
About 70 opposition candidates were on the ballot - a record number - and 477 observers from the Organisation for Security and Co-operation in Europe (OSCE) were monitoring the poll.
However, the other 193 candidates were all Lukashenko loyalists and in 15 of the 110 districts the president's allies were running unopposed.
Opposition leaders urged the observers not to give their stamp of approval to the poll.
"The monitors face a political and ethical dilemma," Vinchuk Vechorko, deputy head of the opposition Belarus National Front, said.
"They can carry out a political project of remaking Lukashenko into a European, nationally minded politician ... or remain committed to their principles of calling a fraudulent election a fraudulent election."
Sergei Kalyakin, leader of Belarus' Communist party, said: "I have no doubts the polls will be falsified."
Anatoly Lebedko, leader of the opposition United Civic Party, said Lukashenko "will tell Russia: give me good gas prices or I will walk away to Europe, and he will tell Europe: deal with me or ... you will see the Russian bear at your doorsteps".
'Democratic without precedent'
Lukashenko, who has ruled the former Soviet nation for 14 years, has voiced hopes that the European Union and the US will recognise the results of the election.
Lukashenko has ruled Belarus for 14 years [AFP]
"We want you to accept us, to endorse and recognise our election," he said last week.
"We do not want to talk to you across the Iron Curtain which you have erected on the borders of Belarus."
He had promised that the vote would be "democratic without precedent" and threatened to cut ties with the West if its observers said it was rigged, as they did with previous Belarussian polls.
The EU has promised to lift travel restrictions on Belarussian leaders and economic aid to the former Soviet republic of 10 million people, if the vote shows progress towards "democratic values".
Najam Abbas, an analyst from the Asian Century Institute, said that democracy-building takes time and therefore "demonising and raging and encircling" and then demanding democratic results by Western powers was unfair.
In August, Belarus released three people viewed by the West as political prisoners, including former presidential candidate Alexander Kozulin. The opposition has also been given more access to television.
But dissidents have dismissed the moves as cosmetic in a country where the opposition is subject to arbitrary arrests and other forms of intimidation.
'Big brother' Russia
Belarus has been a traditional ally of Russia, which heavily subsidises Belarus' Soviet-style economy, but has been seeking a higher price for its support.
Following Russia's conflict with Georgia last month, which led to a deep chill in Moscow's relations with the West, the Russian ambassador to Minsk criticised Belarus for maintaining a "modest silence" on the conflict.
Minsk has declined to follow Moscow's lead in recognising the independence of Abkhazia and South Ossetia, the Georgian breakaway regions at the heart of the conflict.
Andrew Wilson, a Belarus expert at the London-based European Council of Foreign Relations, said the Georgia war shook up the Moscow-Minsk alliance by showing how vulnerable the ex-Soviet republics were to Big Brother Russia.
"Clearly [Lukashenko] was worried that Russia would use a kind of sphere-of-influence power in Belarus as well as in Georgia, particularly economically."
Alexander Milikevich, a former opposition presidential candidate and leading opponent of Lukashenko, said Russia "wants maximum influence on the domestic and foreign policy of the former Soviet republics, of Ukraine, Georgia [and] Belarus.
"It would like to preserve them as satellites," he said.
Kozulin, the recently freed prisoner and former presidential candidate, said Lukashenko had "no strategy for improving relations with the West" and was merely playing both sides "to get the maximum benefit".
Alexander Byalyatsky, head of the Vyesna human rights centre, said: "After the election, there will be a strong wave of repression."