Maria Moroz, an ally of opposition leader Svetlana Tikhanovskaya, has been detained on eve of tense vote.
Police in Belarus have cracked down on opposition protesters after an official exit poll put President Alexander Lukashenko on course to win a sixth term.
The exit poll for state television on Sunday gave Lukashenko 79.7 percent of Sunday’s vote, with his main challenger, Svetlana Tikhanovskaya, coming second with 6.8 percent. A political newcomer, Tikhanovskaya mounted an historic challenge to Lukashenko and drew big crowds in campaign rallies around the country.
Preliminary results are not expected until Monday.
Lukashenko, who has been in power since 1994, had pledged to crush any protests but thousands of protesters took to the streets of the capital, Minsk, shortly after the exit poll was broadcast. The opposition had said it expected the results to be rigged.
Late on Sunday, the atmosphere in the city was tense as riot police faced off with opposition protesters, beating some with truncheons and using flash-bang grenades to try to disperse them.
Al Jazeera’s Step Vaessen, reporting from Minsk, said the capital was “a city filled with anger and frustration” and described “hectic scenes” that “Minsk hasn’t witnessed before”.
“The streets and squares are full of people who are all very angry, calling the election a huge fraud,” added Vaessen.
“They’re very defiant but also very scared because riot police are out in force. They are detaining people right in front of us, so people are spreading around trying to get away from police.”
Police presence in Minsk was heavy throughout the day, and in the evening, police set up checkpoints on the city’s perimeter to check residence permits, apparently worried that protesters would come from other cities.
There were reports of a police vehicle driving into a crowd of protesters, while ambulances were seen picking up demonstrators who were lying on the streets.
There were reports of injuries and dozens of people arrested, though the interior ministry denied any wounded.
News reports also said police fired tear gas at protesters in the city of Brest.
Late on Sunday, Tikhanovskaya, 37, said she did not trust the poll results showing an overwhelming victory for Lukashenko.
“I believe my eyes, and I see that the majority is with us,” she told a news conference. “We have already won, because we have overcome our fear, our apathy and our indifference.”
Tikhanovskaya, an English teacher by training and stay-at-home mother, mounted a surprise opposition campaign against Lukashenko after her husband, a popular blogger, was jailed and barred from running.
Tens of thousands of supporters attended her rallies, and many voters wore the opposition’s trademark white bracelets at polling stations on Sunday. “I want honest elections,” she said outside a polling station in Minsk.
Earlier on Sunday, Tikhanovskaya’s campaign office said one of her key allies, Veronika Tsepkalo, had left for Russia out of concern for her safety.
Tsepkalo’s ex-diplomat husband Valery Tsepkalo was barred from standing. Maria Kolesnikova, campaign chief of ex-banker Viktor Babaryko, was also dropped from the polls and was jailed on Saturday but later released.
Sporadic protests erupted in major cities, including Minsk, after polling stations closed on Sunday evening with long lines of potential voters still waiting to cast their votes.
A video posted by independent news outlet Tut.by showed several police officers beating a small group of protesters before a crowd of civilians intervened.
Witnesses said scuffles broke out with police using stun grenades, the AFP news agency reported.
In Russia’s capital, Moscow, hundreds of Lukashenko opponents gathered outside the Belarusian embassy for a spontaneous protest in which the crowd chanted: “Get out!”
“It is unbearable to have him in power for so many years. The man should understand himself that he must just leave,” said Yuri Kanifatov in Moscow, who voted against Lukashenko.
Political observers had predicted Lukashenko would rig the vote in the absence of international observers. He won more than 83 percent in previous polls in 2015.
“There can be no recognition of such an election result,” Tikhanovskaya’s spokeswoman Anna Krasulina told the DPA news agency.