The EU has "strongly condemned" the death of a Belarusian artist, who died after he was detained by police in the capital Minsk.
A European Union spokesperson has cited reports that the man was beaten by plain-clothes police officers, a claim denied by Belarusian authorities.
Roman Bondarenko was stopped by police on Wednesday in a courtyard of a building that has become an emblematic place for opposition demonstrations, according to a statement from the Belarusian Investigative Committee.
The 31-year-old painter was detained after an altercation between local residents who were hanging red and white ribbons — the colours of the opposition movement in Belarus — and individuals who were removing them, it added.
The committee said that at the time of his arrest Bondarenko had "physical injuries and signs of alcohol intoxication".
He was then taken to a police station before his condition worsened and law enforcement officers called an ambulance, it said.
The Investigative Committee said the man died the next day in hospital despite several hours of medical treatment.
Authorities were investigating to establish the exact cause of the artist's death, it added.
Belarusian media have disputed the claim that Bodnarenko was intoxicated while being treated by doctors.
EU condemns the painter's death
The European Union has denounced the Bondarenko killing and threatened to take further sanctions against the perpetrators of repression in Belarus.
"Roman Bondarenko died ... according to reports, by the brutality of plain-clothed policemen," announced a spokesperson for EU foreign policy chief Josep Borrell on Friday.
"This is an outrageous and shameful result of the actions by the Belarusian authorities who have not only directly and violently carried out repression of their own population, but also created an environment whereby such lawless, violent acts can take place," they added.
The spokesperson added that Belarus had ignored the fundamental rights of citizens and disregarded their lives, as well as strongly condemning the violence against demonstrators.
"The EU stands in solidarity with all the Belarusians who have suffered and continue to suffer at the hands of the Belarusian authorities," they said.
"We expect the authorities to end the violence and persecution, to release immediately and unconditionally all arbitrarily detained persons, including political prisoners, and to investigate fully and transparently all human rights violations and abuses, and hold those responsible to account."
The European Union has already imposed sanctions on 55 individuals for violent repression and intimidation in Belarus.
Belarus' Interior Ministry has denied responsibility for Roman Bondarenko’s death, saying he was killed following a scuffle with "aggressive locals".
Emotional tributes to Bondarenko in Belarus
The announcement of Bondarenko's death on Thursday evening has sparked emotional tributes in Belarus, and dozens of flowers and candles were placed in the courtyard where the incident took place.
A number of opposition accounts on the widely-used Telegram messaging service also changed their profile photos to black as a sign of mourning.
Opposition figure Svetlana Tikhanovskaya denounced the death of an "innocent man" and has called for human chains to be formed in tribute.
"There is nothing more inhuman than terror against one's own people," said Tikhanovskaya in a statement.
"We can honour his name with a minute of silence and chains of remembrance tomorrow at 12:00 in every district of every city."
"Let's do it for the sake of a man who was killed because he wanted to live in a free country."
Linas Linkevicius, the foreign minister of Lithuania, where Tikhanovskaya resides in exile, also tweeted to say he was "shocked" by the news.
"Astonishing cynicism, cruelty of the regime and blunt ignorance of people’s aspirations for changes," he added.
Belarus has been shaken for more than three months by a historic protest movement against President Alexander Lukashenko, following his disputed re-election in August.
Since the start of the protests, at least four people have officially died during demonstrations or after being arrested by authorities.
Thousands of demonstrators have also been detained during large-scale protest marches.