Some of music's biggest names, including Chris Martin, Angelique Kidjo and Damon Albarn, have signed a petition calling for the release of Bobi Wine, a Ugandan MP and pop star.
The opposition politician, whose real name is Robert Kyagulanyi, has been in detention for more than a week.
The musicians have condemned his "vicious" treatment in custody.
Bobi Wine's lawyers say he has been seriously assaulted in detention, which the military denies.
He is due to appear before a military court on Thursday to face charges relating to the illegal possession of firearms.
The petition was started by Rikki Stein, the former manager for Nigerian music legend Fela Kuti - a fearless critic of Nigeria's military regimes during his lifetime.
"It's like a re-run of Fela. He was often beaten for speaking out," he told the BBC.
The more than 80 signatories also include Nigerian Nobel laureate Wole Soyinka, U2 bassist Adam Clayton, The Pretenders lead singer Chrissie Hynde and Genesis lead singer Peter Gabriel.
Why was Bobi Wine arrested?
He was detained, along with more than 30 other people, in the run-up to last week's bitterly fought by-election in the north-western town of Arua.
He and President Yoweri Museveni, who has been in power since 1986, were in the area campaigning for their rival candidates.
The authorities say opposition lawmakers led supporters to attack the president's convoy with stones. Bobi Wine's driver was later shot dead.
The ruling party went on to lose the seat to opposition candidate Kassiano Wadri, who also remains in custody and is facing treason charges.
The authorities say weapons and ammunition were found in Bobi Wine's hotel room in Arua.
The charges are widely viewed as politically motivated and aimed at silencing a prominent critic of the president.
Who is Bobi Wine?
He is a successful Afrobeats star, known for a repertoire of catchy tunes that touch on poverty and social justice issues.
The lifestyle and love life of the self-styled "ghetto president", which refers to his upbringing in the poorest slum of the capital, became tabloid fodder.
He shifted gears in 2017, cutting off his dreads and donning sharp suits as he turned his eye to Ugandan politics.
He went on to beat more established candidates by a landslide in a by-election and became the MP for Kyadondo East in central Uganda.
Why is he so popular?
In a country where more than three-quarters of the population is under the age of 30, Bobi Wine might have demographics on his side.
He has been described as a magnetic presence and a skilled orator, who has the potential to galvanise the country's youth.
He told the BBC in 2017 that he wanted to be the voice of the younger generation. "I am going to stand up for issues. I'm here to give young people confidence," he said.
He might also have a knack of being kingmaker. In the past year, three candidates backed by Bobi Wine have won parliamentary seats in by-elections.
Is he upsetting the status quo?
Yes. Bobi Wine, who campaigns under the slogan "people power", has styled himself as the voice of the ordinary people.
He protested with activists against a social media tax introduced to boost state revenue, which the government has since backtracked on.
Bobi Wine was also one of the leading critics of a push to scrap the constitutional upper age limit, set at 75, for presidential candidates.
But the change was signed into law and will allow 74-year-old Mr Museveni to run for a sixth term in 2021.
Does President Museveni see him as a threat?
Bobi Wine's popularity could make him President Museveni biggest challenger in the 2021 elections and his arrest has only helped to push him into the international eye.
Political tension has risen in the country since his arrest.
His lawyers say Bobi Wine was so badly beaten in military custody that he could barely see, talk or walk when he first appeared before a military court last week - an allegation Mr Museveni labelled as "fake news".
Pockets of protests have nonetheless broken out over his detention and that of several other opposition MPs.
Human Rights Watch has called for independent investigation into the shooting of Bobi Wine's driver.
While the US, Mr Museveni's key ally, has weighed in against reports of "brutal treatment" against journalists, MPs and protesters "at the hands of security forces", according to a statement by the US embassy in Uganda.
On Tuesday, Uganda's army made a rare apology after soldiers were caught on film beating up a journalist.
Dele Sosimi, who started his career as Fela Kuti's keyboard player, told the BBC that he signed the petition about Bobi Wine because "it's alarming to see the rule of law and basic human rights cast aside".