One of the last remaining high-profile opposition figures in Belarus, Maxim Znak, has been detained by masked men, his supporters say.
Mr Znak, a lawyer and member of the opposition Co-ordination Council, failed to show up for a scheduled video meeting on Wednesday morning.
Witnesses reported that he had been led down a street in the capital, Minsk, by masked men in plain clothes.
Several other opposition figures were recently detained in the country.
Maria Kolesnikova, one of three women who joined forces to challenge Alexander Lukashenko in August's presidential election, is currently in detention after she resisted attempts by the authorities to expel her to Ukraine.
She is the last of the three women leading the opposition to remain inside Belarus.
Mr Znak, meanwhile, is believed to be the last active member of the Co-ordination Council inside the country. The body was set up by the opposition to oversee a transfer of power after the disputed election.
The vote triggered mass protests amid allegations of vote-rigging.
What happened to Maxim Znak?
The 39-year-old, who previously worked as a lawyer for jailed presidential candidate Viktor Babaryko, was due to take part in a video call on Wednesday but failed to dial in.
When a colleague called him, Mr Znak said someone had arrived and then hung up.
He then texted the word "masks" to a group, one activist told local media. This is believed to be a reference to the face masks worn by the Belarusian security services.
Witnesses then reported seeing Mr Znak being led down a street close to his offices by masked men in civilian clothes.
On Monday, Mr Znak had told the BBC he was concerned about his safety.
"I'm pretending to be relaxed," he said. "It's a professional habit - but actually I'm very concerned and scared."
The police are yet to comment on reports of his detention.
When opposition figures set up the Co-ordination Council to, as they put it, oversee the transfer of power, there were seven members on the presidium.
Now, all but one have fled the country or been detained after being threatened with arrest.
By picking off key figures, the authorities are clearly hoping to scare others into submission and put a stop to weeks of street protests.
But, from the start, these protests have been organised via social media, not by any one activist, party or politician.
What else is happening?
On Wednesday, the exiled opposition leader Svetlana Tikhanovskaya visited the Polish capital Warsaw where she delivered a speech at a university and called for the protests in Belarus to remain peaceful.
"I think it is impossible to fight violence and give violence," she said.
She has also called for the immediate release of Mr Znak.
"The methods employed by the so-called authorities are outrageous," she said in a statement. "It's clear Lukashenko is afraid of negotiations and is trying... to paralyse the work of the Co-ordination Council and intimidate its members."
Ms Tikhanovskaya, the chief opposition rival to Mr Lukashenko in last month's election, was forced to go into exile in neighbouring Lithuania shortly after the vote.
media captionProtesters took to the streets of Minsk and demonstrated outside President Lukashenko's palace
She entered the race after her husband Sergei Tikhanovsky and another candidate were jailed.
Also on Wednesday, the Belarusian interior ministry said 121 people had been detained at protests nationwide on Tuesday. Hundreds more people were arrested on Sunday during the fourth consecutive weekend of anti-government demonstrations.
Mr Lukashenko, meanwhile, is set to meet Russian President Vladimir Putin on 14 September. The pair will discuss energy co-operation, regional conflicts and many other topics, the RIA news agency reports.