The main challenger to Belarus's Alexander Lukashenko has refused to accept the autocratic president won 80% of the vote in Sunday's election.
"I consider myself the winner of this election," Svetlana Tikhanovskaya said on Monday.
This follows thousands of arrests after protesters and riot police clashed in the capital Minsk and other cities.
A lack of scrutiny – no observers were present – has led to widespread fears of vote-rigging in the poll.
The election was held amid growing frustration at Mr Lukashenko's leadership, with opposition rallies attracting large crowds. The preceding days saw a crackdown on activists and journalists.
The president has described opposition supporters as "sheep" controlled from abroad, and vowed not to allow the country to be torn apart.
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Mr Lukashenko won 80.23% of the vote, according to a preliminary count, with Ms Tikhanovskaya receiving 9.9%.
Ms Tikhanovskaya entered the election in place of her jailed husband and went on to lead large opposition rallies.
Mr Lukashenko, 65, has been in power since 1994.
What did Ms Tikhanovskaya say?
The opposition candidate said that the election results published on Monday morning "completely contradict common sense" and the authorities should think about how to peacefully hand over power.
"We have seen that the authorities are trying to hold on to their positions by force," she said.
"No matter how much we asked authorities not to turn on their own people, we were not listened to."
Her campaign said it would challenge "numerous falsifications" in the vote.
"The election results announced by the Central Electoral Commission do not correspond to reality and completely contradict common sense," her spokeswoman Anna Krasulina said.
But Mr Lukashenko poured scorn on Ms Tikhanovskaya's comments.
"So Lukashenko, who is at the top of the power structure and at the head of the state, after getting 80% of the vote must voluntarily hand over power to them," the president said. "The orders are coming from over there [abroad]."
"Our response will be robust," he added. "We will not allow the country to be torn apart."
What has the international reaction been?
Russian President Vladimir Putin congratulated his Belarusian counterpart on his victory, despite friction over accusations of a Russian plot which Mr Lukashenko has tried to link to the opposition.
The leaders of China, Kazakhstan, Uzbekistan, Moldova and Azerbaijan have sent messages of support.
But the German government said it had "strong doubts" about the election and that minimum standards were not met.
European Commission President Ursula von der Leyen called for the election results to be published.
"Harassment and violent repression of peaceful protesters has no place in Europe," she said.
Meanwhile, Poland has called for an emergency EU summit to discuss the crisis.
'Happy to be a loser'
Belarusian media and online pundits offer starkly different narratives about the poll aftermath.
Government-owned Belarus 24 TV accuses "provocateurs" of "blocking the functioning of polling stations". It says "aggressive youths" acting on instructions received via [messaging app] Telegram tried to seize government buildings.
"The clashes were staged by groups of aggressively-minded young people. Unfortunately, there are victims," says another state channel, STV. "Those guilty of provocation will be held responsible."
The main government newspaper Belarus Segodnya does not mention the protests. Its top headline simply proclaims that "Lukashenko is elected president" and the paper carries congratulatory messages from world leaders.
Although pro-opposition online media have been hit by an internet blackout, there is a lively debate on social media.
"This is war. A war waged by a crazy old man against his people," says well-known journalist Dmitry Halko on Facebook.
"Blood on faces, blood on clothes, blood on the asphalt. If this is victory, then I am happy to be among the losers," says writer Viktor Martsinovich.
What happened in Sunday's protests?
Demonstrators took to the streets in central Minsk as soon as voting ended. Many chanted "Get out" and other anti-government slogans.
Police used stun grenades, rubber bullets and water cannon.
Reports from a human rights group that a man had died proved to be untrue.
However, social media footage showed a man who had clung to the front of a police truck lose his grip as it accelerated, hitting his head.
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