Kizza Besigye has run against Ugandan President Yoweri Museveni in four elections and lost each time, although he has rejected the results, alleging fraud and voter intimidation.
He has been arrested dozens of times before.
"I request the (government) of Uganda to release my husband Dr Kizza Besigye from where he is being held immediately," said his wife, Winnie Byanyima, who is the head of UNAIDS, on Wednesday.
She did not describe the alleged kidnapping and could not immediately be reached for further comment.
I request the govt of Uganda to release my husband Dr Kizza Besigye from where he is being held immediately. — Winnie Byanyima (@Winnie_Byanyima) He was kidnapped last Saturday while he was in Nairobi for Hon Martha Karua's book launch. I am now reliably informed that he is in a military jail in Kampala. We his…November 19, 2024
A Ugandan police spokesperson said police were not holding Besigye.
The Ugandan military could not immediately be reached for comment.
Spokespeople for Kenya's police and UNAIDS did not immediately respond to a request for comment.
In July, Kenyan authorities detained 36 members of Besigye's Forum for Democratic Change (FDC) party, one of Uganda's main opposition groups, and deported them to Uganda where they were charged with terrorism-related offences.
"It is very, very shocking that Kenya, which used to be a safe haven for dissidents, is now increasingly becoming an operational zone for the dictatorship," Bobi Wine, another prominent Ugandan opposition figure, told local broadcaster NTV.
Besigye was Museveni's physician during the guerrilla war of the 1980s that brought the Ugandan president to power but later became an outspoken critic.
He was abducted on Saturday during the launch of a book by veteran Kenyan opposition politician Martha Karua, Byanyima wrote on the social media platform X.
Ugandan authorities have arrested Kizza Besigye dozens of times. (AP PHOTO)
She did not give a description of the people who allegedly detained her husband.
"I am now reliably informed that he is in a military jail in Kampala," said Byanyima, the executive director of UNAIDS, the United Nations agency for HIV/AIDS.
"We his family and his lawyers demand to see him. He is not a soldier. Why is he being held in a military jail?"
Several of Besigye's supporters and party officials gathered at Makindye barracks in Kampala on Wednesday, in anticipation that he would be arraigned before a military court.
Museveni's government has been accused of repeated human rights abuses against opposition activists, including illegal detentions, torture and extrajudicial killings.
Officials deny the accusations and say those arrested are detained legally and processed appropriately by the court system.
In October Kenya extradited four Turkish refugees to Ankara, despite Amnesty International raising concerns they could face persecution at home.
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