Ibrahim Zakzaky, the leader of the Islamic Movement of Nigeria (IMN), was arrested in December 2015 after his supporters clashed with the military in the northern city of Zaria.
He lost the sight in his left eye during two days of unrest, in which troops allegedly killed more than 300 IMN supporters and arrested hundreds of others.
His lawyer and human rights groups have repeatedly called for him to be released on health grounds from secret police detention in the capital, Abuja.
Rumours swirled earlier this week that he had died but on Saturday he was seen with his wife, Zainab, at the offices of the country’s intelligence agency, the Department of State Services.
The 64-year-old cleric wore a neck brace over his traditional robes and walked with the help of a stick before getting into a waiting vehicle.
He told reporters only that an unspecified medical condition had been “severe on me on Monday” but symptoms then subsided and he was allowed to see his own doctor for the first time.
“I’m getting better,” he said.
Zakzaky’s wife is said to have been in a “far worse” state, with bullets from the attack in December 2015 still lodged in her body.
The couple’s lawyer said in November last year that she had been in “excruciating pain and agony on a daily basis” and her life was in danger unless she received urgent medical attention.
Nigeria’s government has previously ignored a court order to release the couple.
Zakzaky has been at loggerheads with Nigeria’s secular authorities before because of his repeated calls for an Iranian-style Islamic revolution in Nigeria.
Northern Nigeria is predominantly Sunni Muslim.
A DSS official, Nnanna Nnochiri, said the rumours about Zakzaky’s health were “unfounded” and circulated by people wanting to “foment trouble”.
“The man is alive, he’s on his feet, he’s hale and hearty, he’s up and doing,” he said.