Thousands of people have been making their way in columns towards the centre of the Belarusian capital Minsk for a new protest against the president.
Riot police, backed by water cannon and barriers, have sealed off parts of the city and a number of arrests have been made, with reports of injuries.
Protesters seek the resignation of President Alexander Lukashenko.
They accuse the authorities of rigging his re-election just over a month ago, sparking deadly mass unrest.
At least four people have died and hundreds have been injured since then, as the government tried to stamp out dissent.
A number of opposition figures have fled the country, On Saturday, activist Olga Kovalkova became the latest to say she had taken refuge in neighbouring Poland amid threats of imprisonment.
Mr Lukashenko, in power since 1994, has accused Western nations of interfering.
Protesters, human rights activists and observers say riot police are brutally suppressing peaceful marches.
The former Soviet republic borders Russia, on which it depends heavily for energy and with which it historically has close ties, as well as Ukraine and EU states.
What is happening at the moment?
Sunday has become the key day for street demonstrations since the rallies began, the BBC’s Jonah Fisher reports from Minsk.
But riot police have intensified their efforts to intimidate and block the flow of people heading into the centre of city today, while detaining those taking part in the demonstrations, our correspondent says.
He adds that in the past few days the security forces – dressed all in black with balaclavas over their faces – targeted university students as they returned from their holidays, dragging some from the streets and university buildings into unmarked minivans.
Pictures on social media on Sunday showed armoured personnel carriers and water cannon vehicles driving into the centre of Minsk, some heading for Independence Square, which has been cordoned off.
One protester in Minsk who gave her name as Lyudmila told BBC News the demonstrators were undeterred by the security forces.
“We definitely not ready to get back to the life we had for many years now,” she said.
“We finally feel like we matter because we’ve been living in apathy for way too long and now we just have this feeling of solidarity and we actually think that – well, I feel personally that – changes already are happening so that’s definitely not the time to give up.”
Protests are also being reported in other Belarusian cities and towns including Grodno, Mogilev and Gomel.
What happened to Olga Kovalkova?
The activist said on Saturday she had left for Poland as she would have faced a long jail term had she not agreed to leave Belarus.
Ms Kovalkova said security forces had driven her to a border post where she was able to board a bus to Poland after the driver recognised her.
A spokesman for Polish Prime Minister Mateusz Morawiecki said his country would offer support to victims of repression in Belarus.
Ms Tikhanovskaya, 37, represented the chief opposition to Mr Lukashenko in the election, entering 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.
How has the EU reacted to events in Belarus?
Last month, EU leaders agreed to impose sanctions – 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.”
What is Lukashenko’s stance?
Mr Lukashenko has denied any allegations of vote-rigging. He has blamed some EU nations, in particular Poland and Lithuania, of trying to force regime change.
The 66-year-old has promised to defend Belarus.
On Thursday, he indicated he was open to moving closer to Russia, saying the protests had “prompted us to make relevant conclusions”.
On at least two occasions he has been photographed near his residence in Minsk carrying a gun and being surrounded by his heavily armed security personnel.