A former colleague of opposition journalist Roman Protasevich, who was arrested in Belarus after being taken off a diverted flight on Sunday, says dissidents now fear for their lives.
Stepan Putilo told the BBC that he had received death threats in the past, but now he was “taking them seriously”.
Mr Putilo founded the opposition social media channel Nexta on Telegram, where Mr Protasevich had acted as editor.
Mr Protasevich and his girlfriend Sofia Sapega are both in custody in Belarus.
The journalist, 26, has said he fears the death penalty after being placed on a terrorism list.
Mr Protasevich and Ms Sapega, a 23-year-old Russian national, were detained after Belarus scrambled a military jet to escort their plane – flying from Athens to Vilnius, in neighbouring Lithuania – to Minsk, the Belarusian capital.
Western countries accuse Belarus of hijacking the Ryanair plane that was rerouted over a supposed bomb threat.
Addressing MPs on Wednesday, Belarusian President Alexander Lukashenko defended the decision to divert the flight.
“We need to take appropriate measures to protect our country,” he said, adding that he had acted within the law.
During his speech, Mr Lukashenko did not mention the arrest of Mr Protasevich.
The journalist’s lawyer, Inessa Olenskaya, told the BBC she has not been allowed to speak with him yet.
“At the moment I haven’t been shown a single document or told anything. I didn’t even get past the reception desk… I’ve spent the last two days on chairs in the entrance,” she said.
‘We show what they want to conceal’
Nexta, a channel on the messaging app Telegram that has more than a million subscribers, was used for mobilising street protests last year.
Mr Putilo told the BBC that the Belarus government under Mr Lukashenko “fears us because we show the truth”.
“We show what they want to conceal,” he said. “If the regime cares enough to bring down Roman’s plane, then we are doing something right, and we will carry on fighting.”
“Roman is more in danger than other political prisoners in Belarus,” he said, speaking from the Polish capital, Warsaw, adding: “He is Lukashenko’s personal enemy.”
A duty that carries serious risk
The BBC’s Jean Mackenzie reports from the Nexta headquarters in Poland
Hundreds of miles from Minsk, half a dozen young activists, all in exile, are working to keep Nexta’s social media platforms running.
Two police officers guard the entrance to their dilapidated office. Roman’s old desk is empty. Although he left in November, it now serves as a reminder of the risk they are all taking.
Most of the employees do not want their identities to be known. They have parents back home in Belarus, who they need to protect. But they feel this is their duty, to keep fighting for the country’s democracy.
Nexta’s founder Stepan is younger than Roman, having fled Belarus when he was only 20. He checks his phone constantly, hoping for news of his friend and former colleague, but there is none. Stepan knows he is just as much of a target as Roman – if not more. They had felt relatively protected here in exile, but that has changed. Now, not even the streets of Warsaw feel safe.
Mr Putilo’s comments come after the families of Mr Protasevich and Ms Sapega – who have both been filmed giving statements in detention – spoke of their concern for the pair’s safety.
“I’m calling on the whole international community to save him,” Mr Protasevich’s mother, Natalia, said.
In Ms Sapega’s video statement, she says she edits a Telegram channel that publishes personal information of Belarusian policemen. However, it is likely she is speaking under duress.
“Today Sofia was interrogated. She was accused of committing a criminal offence. A preventive measure was chosen – detention for a period of two months,” her lawyer, Alexander Filanovich, told the BBC’s Russian service on Tuesday.
Ms Sapega is now in a pre-trial detention centre in Minsk, he said.
Ms Sapega’s mother cast doubt on how freely her daughter was speaking in the video, released by a pro-government Telegram channel.
“She sways, eyes in the sky – as if afraid of forgetting something,” she told the BBC.
“I enlarged [the video] as much as possible – it seems that [she looks] okay. We are now packing warm clothes, we will go to Minsk. I want to try to give her a parcel – I saw she only had a thin jacket.”
On Monday, Belarusian authorities released video of Mr Protasevich that appears to have been recorded under duress.
Natalia Protasevich told AFP news agency that she had not slept since her son’s arrest.
“I’m asking, I’m begging, I’m calling on the whole international community to save him,” she said, breaking down in tears during an interview in Wroclaw, southern Poland.
“He’s only one journalist, he’s only one child but please, please… I am begging for help. Please save him! They’re going to kill him in there!”
She added that her son was a “fighter for justice”.
“They sent a fighter jet to get this young man! It’s an act of terrorism, I don’t think you can call it anything else. He’s been taken hostage. This is an act of pure revenge!” she said.
Her voice breaking, she added: “My son, this young man, just wanted to tell the truth about the situation. He didn’t do anything wrong.”
How were the couple arrested?
Belarus sent a fighter jet to escort Ryanair flight FR4978, claiming there was a bomb threat. It touched down in Minsk at 13:16 local time (10:16 GMT) on Sunday.
Police then took Mr Protasevich away when the plane’s 126 passengers disembarked. The activist, who witnesses said was “super scared”, was arrested along with Ms Sapega.
On Tuesday, the Belarusian transport ministry released a transcript of a conversation said to be between an air traffic controller in Minsk and a pilot on the Ryanair flight.
According to the transcript, which has not been independently verified, Belarus suggested several times that the plane should land in Minsk on “our recommendation”.
This appeared to contradict earlier statements from the Belarusian authorities that said the decision to land was made independently by the pilot.
Belarus: The basics
Where is Belarus? It has its ally Russia to the east and Ukraine to the south. To the north and west lie EU and Nato members Latvia, Lithuania and Poland.
Why does it matter? Like Ukraine, this nation of 9.5 million is caught in rivalry between the West and Russia. President Lukashenko has been nicknamed “Europe’s last dictator” – he has been in power for 27 years.
What’s going on there? There is a huge opposition movement demanding new, democratic leadership and economic reform. The opposition movement and Western governments say Mr Lukashenko rigged the 9 August election. Officially he won by a landslide. A huge police crackdown has curbed street protests and sent opposition leaders to prison or into exile.