Italy seized more than 20 tonnes of cannabis on Thursday from a ship sailing off the island of Sicily.
Coastguard officers escorted the boat to the Sicilian port of Marsala and discovered the hashish resin in burlap sacks, police said in a statement. Six crew members of Syrian origin were taken into custody.
"This is the most important operation in the Mediterranean Sea in the past 10 years," Cristino Alemanno, of the Guardia di Finanza, said.
The ship had left the North African state of Morocco, one of the world's largest producers of illicit hashish, mostly shipped to Western Europe.
Authorities said they had monitored the ship for two days as it entered Italian waters and officers boarded the ship after a request to the Comoros islands where the ship was registered was not answered.
On Saturday, Italian authorities seized a tonne of hashish from a Turkish ship in a separate operation.
--Reporting by Naomi O'Leary; Editing by Alison Williams (C) Reuters 2013.