Eddie Mutwe, who also acts as the chief bodyguard for Uganda's leading opposition figure, Bobi Wine, went missing last weekend after being grabbed near the capital Kampala by armed men, the National Unity Platform (NUP) party has said.
The police have said they do not have Mutwe, whose real name is Edward Ssebuufu, and until now there had been no word on his whereabouts.
In a series of posts on X late on Thursday, Chief of Defence Forces Muhoozi Kainerugaba posted what appeared to be a photograph of Mutwe, who was shirtless, and said he had captured him "like a grasshopper".
Reuters was not immediately able to independently verify the photograph, but the NUP party later re-used it on their X handle in a post seeking support for Mutwe.
"He is in my basement ... You are next!," Kainerugaba wrote in a post responding to one from Wine about Mutwe's disappearance.
"I still have to castrate him," he said a few hours later, adding that he would release Mutwe only when Museveni gave the order.
The Uganda Human Rights Commission, a government body, on Thursday ordered Kainerugaba to release Mutwe, who they said had been unlawfully detained since April 26, or he would be "liable for contempt under ... the constitution".
Spokespeople for the Ugandan government, military and police did not immediately respond to requests for comment.
Uganda's opposition has accused Museveni of fast-tracking Kainerugaba's military career to prepare him to eventually succeed to the presidency, despite his son's frequent inflammatory remarks on social media, including threats in 2022 to invade neighbouring Kenya and in January to behead Wine.
Wine is a popular musician-turned-politician who came second in the 2021 election.
Museveni, 80, has ruled Uganda since 1986 and is expected to stand for re-election next January. His opponents and human rights activists have regularly accused his government of wide-ranging abuses including abductions and illegal detentions.
Museveni has denied these allegations.
"Eddie Mutwe's ordeal is not an isolated incident but part of a systematic campaign to silence dissent and crush the aspirations of young people yearning for freedom," the Uganda Law Society said in a statement.
Wine, who says the 2021 vote was marred by massive fraud, wrote on X on Friday afternoon that soldiers had just raided his party headquarters ahead of a planned gathering to support Mutwe.
Kizza Besigye, another opposition leader, who challenged Museveni in four elections, was detained in November and remains in jail on charges including treason.
