Vitaly Shishov’s body was found hanged in a park in Kyiv, a day after he failed to return from a jog. Police have opened a murder inquiry.
Police said they were investigating whether he had been killed and his death made to look like suicide.
Meanwhile, a Belarusian Olympic sprinter who feared for her safety has been granted a humanitarian visa by Poland.
Mr. Shishov led the Belarusian House in Ukraine (BHU), a group helping people who left Belarus, where opposition to the government is stifled.
The United Nations said Mr Shishov’s death adds another level to “our worries about what is happening in Belarus”, and called for an investigation.
Mr. Shishov’s death is the latest incident to prompt international scrutiny of President Lukashenko’s authoritarian government.
At the Tokyo Olympics, sprinter Krystina Timanovskaya, 24, was granted a humanitarian visa by Poland after refusing her team’s order to fly back to Belarus early.
She said she had been forcibly taken to the airport for criticising team coaches, and voiced fears for her safety.
Police said that they had recovered Mr. Shishov’s mobile phone and personal items from the scene.
They are asking anyone who knew him to get in touch with any information about the last few weeks of his life, his state of mind, and any possible threats to his life.
Mr Shishov left his home on Monday morning and it was assumed he had gone for his daily jog as his running gear was not found there later, his colleagues said.
They conducted their own search of the woods where he usually went running.
Mr. Shishov had felt under constant surveillance since he left Belarus last year, and had recounted instances of being followed recently, they said.
“They said we should watch ourselves because a Belarusian KGB [secret police] network was active here and anything was possible,” Mr Shchuchko said.
The BHU said in a statement: “We were warned repeatedly by local sources and our people in Belarus about possible provocations, going as far as kidnapping and assassination. Vitaly reacted to those warnings with stoicism and humour.”
Crackdown on opposition
The disputed presidential election last August was the catalyst for a harsh crackdown on opposition to Mr. Lukashenko’s government. He has been in power since 1994.
Huge street protests continued for weeks after the disputed vote, which Belarus dissidents and Western governments say was rigged in Mr. Lukashenko’s favour.
The protests were broken up brutally by police, and thousands were detained.
In recent months, Belarusian authorities have been trying to snuff out remaining dissents, jailing university students and shutting down independent media.
Opposition leader Svetlana Tikhanovskaya claimed victory in the election after standing in place of her politician husband, who was detained in March 2020.
She was forced to flee the country with her children the day after the election and now lives in exile in Lithuania.
On Tuesday, Ms. Tikhanovskaya flew to London to discuss the situation in Belarus with UK Prime Minister Boris Johnson.
She told the BBC she would be pushing for “multiple points of pressure” to be applied to the Belarusian government, including tighter sanctions.
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.
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.