Mahmudi, Libya's premier until the final weeks of Moamer Kadhafi's regime, was held under a warrant from the new authorities in Tripoli, and a Tunisian court is due to review their extradition request on Tuesday.
In the letter sent last Wednesday, "we said that Amnesty International believes that if he would be returned to Libya, he would at present face real risks, serious human rights violations, including torture ... extra-judicial execution and unfair trial," the group's north Africa spokesman James Lynch told AFP.
"It's about the risk to the person, it's not about his particular crimes. It's about the risks to him and the people like him being returned," he said.
Shortly after Mahmudi's arrest, a Tunisian court sentenced him to six months in prison after finding him guilty of illegal entry. That decision was overturned on appeal.
Tunisia in August recognised Libya's National Transitional Council as the country's new authority and has committed itself to cooperation on security issues.
But Mahmudi's lawyer Mabrouk Kourchid said this week that his client fears for his life as the sole holder of Libyan state secrets since Kadhafi's death on October 20.
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