Police say they have arrested 29 people on suspicion of smuggling drugs, dismantling a criminal organization that brought large quantities of hashish into Spain from Morocco.

The investigation began in 2014 after police detected a drug-smuggling ring based near Cadiz in southwestern Spain and culminated in raids after information was received mid-May that a shipment was about to take place.

Police say they seized 4.7 metric tons (5.2 short tons) of hashish, six rigid inflatable boats equipped with powerful outboard motors, seven vehicles and several firearms.

One of the addresses raided was located on the Guadarranque River between Algeciras and Gibraltar and had a private jetty. Among those arrested are the alleged ringleader, speedboat pilots and a lookout.

Thursday's statement by police didn't specify when the arrests were made.