Sabrina de Sousa, 60, was convicted in absentia by an Italian court in 2009 over the kidnapping of Abu Omar from a Milan street in an operation allegedly led jointly by the CIA and the Italian intelligence services.
The trial of de Sousa and 22 others secured what were the first legal convictions in the world against people involved in the CIA's extraordinary renditions programme that followed the September 11th, 2001 attacks.
De Sousa, who has dual Portuguese-US nationality, was arrested Monday and was due to be taken to a jail in the northern city of Porto, her lawyer Manuel Magalhaes Silva told AFP.
“Extradition will go ahead in the next few days,” he said.
Portuguese judicial authorities in January 2016 ordered that she be extradited to Italy, but a series of appeals delayed the procedure.
She was initially arrested at Lisbon airport in October 2015 under a European warrant after the Italian court convicted her, 21 fellow CIA operatives and a US soldier over the kidnapping.
Omar, who had been given political asylum in Italy in 2001, claimed he was tortured after being flown to Egypt via Germany in a case that highlighted the controversial secret renditions of suspected radicals by the United States and its allies.
In 2012, the Italian court of appeal confirmed its 2009 convictions, leaving de Sousa facing four years in jail.
De Sousa says she served as an interpreter for the CIA team that organized Omar's abduction but denies any direct role in the operation.