The Portuguese authorities have detained a former undercover C.I.A. officer whose extradition has been sought by Italy, where she faces a prison sentence over her role in the 2003 kidnapping of an Egyptian cleric as part of a secret United States rendition program.

The former officer, Sabrina De Sousa, 60, was detained Monday night and is awaiting imminent extradition to Italy, one of her lawyers, Dario Bolognesi, said.

Ms. De Sousa was among 26 Americans convicted of grabbing the cleric, Osama Moustafa Hassan Nasr, also known as Abu Omar, in Milan in February 2003. The operation was one of the so-called renditions ordered by the George W. Bush’s administration after the terrorist attacks of Sept. 11, 2001. Suspects were captured and taken to other countries for interrogation. The program was ended by the Barack Obama administration.

Mr. Nasr was taken to a military base before being moved to Egypt, where he said he was tortured. In 2013, an Italian court sentenced him in absentia to six years in prison for terrorism-related activities.