Large protests are expected to be held in Belarus later calling for long-time President Alexander Lukashenko to step down as a crackdown intensifies.
On Saturday, security agents in the capital Minsk detained dozens of people, mostly students, in the fourth weekend of protests.
In a separate march, thousands of women chanted “hands off the children”.
Meanwhile, a top opposition activist said she had taken refuge in Poland after threats from security forces.
Demonstrators say the presidential election of 9 August was rigged to keep Mr Lukashenko in office. Protesters, human rights activists and observers have accused riot police of brutally suppressing peaceful marches.
Mr Lukashenko, in power since 1994, has accused Western nations of interfering.
The protests on Sunday may be the largest yet in Minsk since the demonstrations began four weeks ago, the BBC’s Jonah Fisher in Minsk reports.
Riot police have intensified their efforts to intimidate and block the flow of people heading into central Minsk while detaining those taking part in the demonstrations, our correspondent adds.
Demonstrations that were first triggered by a disputed election are now just as much about the thuggish beatings and abuses that have followed.
On the eve of the protests, masked security agents dragged students off the streets of Minsk and bundled them into police vans during a demonstration. Dozens of protesters were detained.
Also on Saturday, leading opposition activist Olga Kovalkova said she had taken refuge in Poland, adding that she would have faced a long jail term had she not agreed to leave Belarus.
She added that security forces drove her to a border post where she was able to board a bus to Poland after the driver recognised her.
Ms Tikhanovskaya, 37, represented the chief opposition to Mr Lukashenko in the election – she entered the presidential race after her husband Sergei Tikhanovsky and another candidate were jailed.
She said the opposition was demanding an end to the police violence, the immediate release of all political prisoners, and a free and fair election.
– including asset freezes – on as yet unnamed Belarusian officials involved in alleged election-rigging, brutality and imprisonment of protesters. The exact sanctions are still being worked out.
The UN special rapporteur on Belarus, Anais Marin, said Mr Lukashenko’s re-election as president was “completely manipulated” and “people’s votes were stolen”.
She accused the Belarus police of torture, citing as one example a 16-year-old who was “so badly beaten up he was left in a coma”.
“The authorities must release all those arbitrarily arrested,” she said. “The government is waging an insane war against its own people.”
Mr Lukashenko has blamed some EU nations, in particular neighbouring Poland and Lithuania, of trying to force regime change.
The 66-year-old, whose key backer is Russia, has promised to defend Belarus. He was recently seen near his residence in Minsk carrying a gun and being surrounded by his heavily armed security personnel.