A long-time ally of former Italian prime minister Silvio Berlusconi facing the final appeal to his conviction for Mafia association has been detained in Lebanon.

Marcello Dell'Utri is in police custody and Italy is preparing to seek his extradition, interior minister Angelino Alfano said.

A former executive in Berlusconi's media empire and a senator in his conservative movement, Dell'Utri was convicted of acting as a mediator between Berlusconi and the Mafia. Dell'Utri denies having ties to the Sicilian Mafia, and any link between it and Berlusconi.