Belarusian opposition leader Svetlana Tikhanovskaya has said she had taken a "very difficult decision" to leave the country, after disputing an election she claimed was rigged.
Ms Tikhanovskaya said she did it for her children as protests continued for a second night. One person has died.
She was "safe" now in Lithuania, the country's foreign minister said.
Poll results gave President Alexander Lukashenko 80% of the vote, but there have been numerous claims of fraud.
Violent clashes between police and protesters over the two nights since the election was held have left at least one person dead, and there have been numerous reports of police brutality.
Ms Tikhanovskaya is said to have galvanised the opposition, which presented a stronger than ever challenge to the autocratic Mr Lukashenko, who has ruled the former Soviet country since 1994.
The president, however, has described opposition supporters as "sheep" controlled from abroad.
In an emotional video address to supporters (in Russian) recorded before her departure for Lithuania and posted on YouTube, Ms Tikhanovskaya said she had overestimated her own strength.
"I thought that this campaign had really steeled me and given me so much strength that I could cope with anything," she said.
"But I guess I'm still the same weak woman that I was."
She said her decision to leave was taken "completely independently" and not influenced by anyone around her, even though many people would "condemn" or "hate" her for it.
"No one life is worth what is happening now," she added. "Children are the most important things in our lives."
Ms Tikhanovskaya sent her children to Lithuania for safety before the election.
Earlier Lithuanian Foreign Minister Linas Linkevicius tweeted about Ms Tikhanovskaya's whereabouts after rumours that she had disappeared.
Skip Twitter post by @LinkeviciusL
Svetlana #Tikhanovskaya is safe. She is in #Lithuania. pic.twitter.com/6f9U2meoX0
— Linas Linkevicius (@LinkeviciusL) August 11, 2020
End of Twitter post by @LinkeviciusL
There had been concern for her on Monday but her campaign later said she was "safe", without saying where.
Mr Linkevicius told Lithuanian radio Ms Tikhanovskaya had been detained for seven hours in Belarus but did not say why or by whom.
An associate of the opposition leader said she had been escorted from the country by the authorities as part of a deal to allow the release of her campaign manager, Maria Moroz, who was arrested on Friday evening. The two women left the country together.
Who is Svetlana Tikhanovskaya?
The election campaign saw the rise of Ms Tikhanovskaya, 37, a former teacher who was a stay-at-home mother until she was thrust into the political spotlight.
After her husband was arrested and blocked from registering for the vote, she stepped in to take his place.
President Lukashenko has dismissed Ms Tikhanovskaya as a "poor little girl", manipulated by foreign "puppet masters".
The political novice challenging an authoritarian president
After the vote took place, her campaign said the results, which gave her just 9.9% of the vote, "did not correspond to reality" and vowed to challenge "numerous falsifications".
Ms Tikhanovskaya told reporters that she had in fact won the election, and called on the authorities to relinquish power peacefully. Protests began as soon as polls closed.
However, Mr Lukashenko said he would respond robustly to the protests and not allow the country to be torn apart.
A symbol of change, not a leader
Svetlana Tikhanovskaya went missing after lodging an official complaint about the election result. She was quoted as saying "I've made my decision" but nobody could confirm her whereabouts for many hours.
Now the foreign minister of neighbouring Lithuania says she is there - and safe. How that happened is not clear yet.
On Monday, the KGB security service in Belarus claimed it foiled a plot to assassinate the opposition candidate - and make her a "sacrificial lamb", for protesters. At a news conference in Minsk, she seemed nervous, slightly unsure; the same day she told the BBC she was scared.
The fact Svetlana Tikhanovskaya has fled, though, will not affect the unprecedented mass protests that have rocked Belarus for a second night - crowds clashing with riot police.
They are organised through social media - mainly Telegram - not by her campaign team and the candidate had not joined them in person. She only ran for president after her activist husband was arrested - and for voters, Svetlana Tikhanovskaya has always been a symbol of change, a route to that, rather than a leader.
What happened in Monday's protests?
Riot police fired rubber bullets, tear gas and stun grenades to disperse thousands of demonstrators rallying in the capital.
The brutality of the crackdown has shocked observers - many of these weapons had not been used in Belarus before the weekend.
Polish-based broadcaster Belsat TV aired footage of the police charging into the crowds.
Protesters clashed with riot police in Minsk
Reports say some of the demonstrators fought back, throwing petrol bombs. Protesters also tried to build barricades.
Officials say one demonstrator died when an explosive device went off in his hands - the first confirmed fatality since the clashes began.
A number of people were arrested. One journalist was injured, her colleagues and eyewitnesses said.
The BBC's Abdujalil Abdurasulov in Minsk says protesters were put in police vans, and the sound of beatings could be heard as the officers entered, and the people inside cried for help.
The scale of the protests and the violence used to disperse the crowds is unprecedented, he says, and protesters are struggling to learn the whereabouts of missing friends and relatives.
Protests were also being held in other Belarusian cities.
The internet, which was "significantly disrupted" on election day, continued to be mostly unavailable for a second day, according to online monitor NetBlocks.
What's the context?
President Lukashenko, 65, was first elected in 1994.
Mr Lukashenko cast his ballot at a polling station in Minsk
In the last vote in 2015, he was declared winner with 83.5% of the vote. There were no serious challengers and election observers reported problems in the counting and tabulation of votes.
Anger towards Mr Lukashenko's government this time has been in part fuelled by its response to coronavirus.
The president has downplayed the outbreak, advising citizens to drink vodka and use saunas to fight the disease.
Belarus, which has a population of 9.5 million, has reported nearly 70,000 cases and almost 600 deaths.