Police took Sabrina de Sousa, who was born in India, to a Portuguese jail where she is awaiting extradition, her Portuguese lawyer, Manuel Magalhaes e Silva, told the Associated Press.
He said in an email that she was detained Monday and is expected to be sent to Italy within days.
De Sousa was among 26 Americans convicted for kidnapping suspect Osama Moustafa Hassan Nas, also known as Abu Omar, from a Milan street on February 17, 2003. She denied involvement in the abduction.
De Sousa lost several appeals against extradition since her arrest at Lisbon Airport in October 2015 on a European warrant. She had argued that she was never officially informed of the Italian court conviction and couldn't use confidential US government information to defend herself.
De Sousa holds both US and Portuguese passports, has said she had been living in Portugal and intended to settle there. She was on her way to visit her elderly mother in India with a roundtrip ticket when she was detained.