Anicet Ekane's death was announced by his lawyer on Monday.
Cameroon: Rival of President Biya dies in detention
He had been in detention since his arrest on October 24 as Cameroon clamped down on post-election protests.
He was accused of supporting Issa Tchiroma who had declared himself the winner of the presidential election. The authorities considered Tchiroma's announcement subversion.
Dozens of people were killed in the crackdown that ensued.
Tchiroma has since sought exile in Gambia.
Ekane's African Movement for New Independence and Democracy (Manidem) party had on November 21st accused the police of confiscating Ekane's medical equipment.
It said his oxygen concentrator had remained in Ekane's vehicle which was seized by the police in the city of Douala.
The party called the confiscation 'gravely dangerous' for its leader's health. Ekane was a major figure in Cameroon's leftist opposition.