A Nigerian man has been sent to a mental institute in Kano state after he declared that he did not believe in God, according to a humanist charity.
Mubarak Bala was being held against his will at the hospital after his Muslim family took him there, it said.
The hospital said it was treating Mr Bala, 29, for a "challenging psychological condition", and would not keep him longer than necessary.
Kano is a mainly Muslim state and adopted Islamic law in 2000.
The International Humanist and Ethical Union (IHEU) says that when Mr Bala told relatives he did not believe in God, they asked a doctor if he was mentally ill.
Despite being told that he was not unwell, Mr Bala's family then went to a second doctor, who declared that his atheism was a side-effect of suffering a personality change, the group says.
Mr Bala, a chemical engineering graduate, was forcibly committed to a psychiatric ward at the Aminu Kano Teaching Hospital, but was able to contact activists using a smuggled phone, it says.
IHEU spokesman Bob Churchill said the group was concerned about his "deteriorating condition" and called for his "swift release".
The hospital said in a statement that Mr Bala was "comfortable and conscious".
He had been admitted because he required treatment under supervision, it added.
Mr Bala's lawyer Mohammed Bello told the BBC he intended to arrange for an independent psychological evaluation of his client because of conflicting accounts of his health.