Uganda's Kyadondo East MP Robert Kyagulanyi, alias Bobi Wine, who faces a court martial in Gulu on Thursday, has hired international law firm Amsterdam & Partners LLP.
The independent lawmaker, who was first presented in court on August 16, faces charges of illegal possession of firearms and ammunition, even though he was originally arrested on claims of obstructing the motorcade of President Yoweri Museveni during campaigns in Arua Municipality.
Lawyer Robert Amsterdam, the founding partner of Amsterdam & Partners LLP, described his client's arrest as a "politically motivated act of repression" and alleges that he has been tortured in custody.
"Mr Kyagulanyi's arrest was not only a politically motivated act of repression, but furthermore he has since been subjected to inhuman and horrific acts of torture by the Ugandan security services from which it is possible he may never recover," he said in a statement.
"This outrageous and reprehensible act of state aggression-- an episode not seen since the days of Idi Amin-- requires a robust and broad response from the international community to secure his safety and to bring his torturers to account."
The army, which is holding Mr Kyagulanyi, denies that he has been tortured and President Yoweri Museveni has dismissed the claims as "fake news".
Mr Kyagulanyi's arrest, along with 34 other people, took place around the same time that Mr Museveni was campaigning in Arua, allegedly after protesters had thrown rocks at the president's car.
A night before he was arrested, his driver was shot dead by soldiers or presidential guards.
He has been held since last week, along with four fellow MPs and dozens of others.
On Wednesday, Uganda Deputy Speaker Jacob Oulanya visited Mr Kyagulanyi at the Makindye Military Barracks.
Mr Oulanyah confirmed the MP's poor state of health.
"I talked with Hon Kyagulanyi but he is in a lot of pain. He is however in a uniquely humorous mood and we were laughing with him," Oulanyah is quoted as saying.
In a eight-second clip circulated widely on social media, Kyagulanyi is seen in a red shirt and pair of shorts with slippers sitting around a plastic table laughing but with apparent difficulty.
Mr Amsterdam is recommending that the US should "hit [government] officials in their wallets with a Magnitsky-style sanctions schedule as they need to understand the consequences of this attack on human rights".
The Global Magnitsky Act, which was passed by the US Congress with broad bipartisan support in 2016 in honour of the murdered Russian whistleblower-lawyer Sergei Magnitsky, empowers the US government to impose visa bans and targeted sanctions on individuals responsible for human rights violations anywhere in the world.
"The situation for Mr Kyagulanyi is profoundly grave, immediate, and deserving of urgent action and intervention," said Mr Amsterdam.
"When a state behaves with this level of impunity and violence, there are few statements or resolutions that will stop them from the murder of an innocent opponent like Bobi Wine."
-- Additional reporting by Charles Mpagi in Kampala.