Police in Agrigento said several migrants identified the 20-year-old as the man who had raped several women and beat, burned or administered electric shocks to migrants who were kept in a "safe house" in Libya awaiting passage.
A police statement said the man arrived on the Sicilian island of Lampedusa on March 5. Authorities said migrants recognized him as having committed torture and they had to intervene to stop several from trying to lynch him.
The alleged torturer, who wasn't identified in the statement, is being investigated for human trafficking, kidnapping, sexual violence and aiding and abetting clandestine immigration.
Other migrants who have reached Italy after rescue at sea in recent years have given authorities similar accounts of being tortured while spending weeks or months in Libya waiting for passage after long land journeys across Africa, where they are fleeing poverty, persecution or violent conflict.
Police said the migrants said the Ghanaian man would torture them while they were speaking by phone to relatives as a way to extort more money from family members, who, alarmed by their screams, would send additional funds to ensure the migrants would soon leave Libya.
The police statement quoted one migrant as saying that whenever he called home, the Ghanaian man would tie him up, make him lie down and beat the soles of his feet with a rubber tube, to the point he could barely walk. Another migrant said the alleged torturer attached electrodes to his tongue and set off an electric current. Others reported being burned by boiling water he poured on them.
Palermo-based prosecutors issued an arrest warrant for the man on March 14.
With much of Libya essentially lawless after the 2011 demise of longtime dictator Muammar Gaddafi, authorities in Italy have heavily relied on migrants' accounts to learn about human traffickers and their helpers.