Tension ran high after 10,000 protesters, unprecedented numbers for the tightly-controlled ex-Soviet state, defied warnings and massed in the city centre of Minsk, the capital, to support challenger Alexander Milinkevich after polls closed on Sunday evening.
They dismissed as blatant fraud results that gave Alexander Lukashenko more than 80% of the vote.
Meanwhile, Alcee Hastings, a top official of the Organisation for Security and Cooperation in Europe (OSCE), told a news conference in Minsk: "The March 19 presidential election does not meet the required international standards for free and fair elections."
Lukashenko, described by Washington as Europe's last true dictator, defiantly said on Monday: "Let me say that the revolution that so many people talked about and some were preparing, has failed and it could not be otherwise."
Referring to Western support for the political opposition, he said: "Despite the pressure and orders from outside, they couldn't break us."
But Milinkevich, credited with just 6% of the vote, called for larger numbers to gather in the city centre later on Monday.
"We consider that there will be an illegitimate president in power ... The only way to talk to these authorities is from the streets. We must take to the streets," Milinkevich told a news conference.
Russia, in comments that were likely to put it at odds with the US and its other G8 partners, voiced backing for Lukashenko despite the Kremlin's distaste for his political style.
"Let me say that the [opposition] revolution that so many people talked about and some were preparing, has failed"
Belarussians had "clearly expressed their will and that must be treated with respect", said Russia's Foreign Ministry.
The European Union responded to Sunday's results by saying it would impose sanctions against Belarus.
Benita Ferrero-Walder, the EU external affairs commissioner, told a news briefing: "From what we have already seen, my view is that some action is very likely."
On Monday, OSCE's Hastings said in Minsk: "It's clear that the incumbent president allowed the state authorities to be used in a manner which did not allow free and fair elections in Belarus.
"While there was competition, it did not exist in an environment for free elections. This was not a level playing field," said Hastings, who was co-ordinator of the OSCE monitors observing the polls and head of the organisation's parliamentary assembly.
The EU's Walder (L) and other
ministers may penalise Belarus
Earlier, EU foreign ministers met in Brussels to ponder sanctions, expected to involve a visa ban on Belarus officials deemed responsible for any election rigging. Diplomats said a final decision could come in April.
"I don't think we are thinking of economic sanctions ... extending visa bans will certainly be one part of it," Ferrero-Waldner said, adding that any possible sanctions should not harm the general population.
Current EU president Austria said the Belarus elections were marred by intimidation.
Ursula Plassnik, the foreign minister of Austria who also led the EU ministers' meeting, said: "The climate of intimidation and hindering of the opposition to do their work was upsetting."