The Turin special police force searched several properties across northern Italy as part of an investigation into Italians who had fought alongside Russian-backed separatist forces in eastern Ukraine, a police statement said.
Three men were arrested in the operation.
Italian media named those arrested as Fabio Del Bergiolo, 50, an Italian ex-customs officer who had reportedly run unsuccessfully in 2001 as a Senate candidate for the neo-fascist Forza Nuova party; Alessandro Monti, 42, a Swiss national; and Fabio Bernardi, 51, also Italian.
During the raids, police seized a French-made Matra air-to-air missile that apparently once belonged to the Qatar armed forces.
Subsequent checks showed the weapon was in working condition but lacked an explosive charge.
According to police, intercepted phone calls indicated that the men were intending to sell he missile.
Other weapons discovered include 26 guns, 20 bayonets, 306 gun parts, including silencers and rifle scopes, and more than 800 bullets of various calibres.
The arms were primarily from Austria, Germany and the United States.
Police also seized neo-Nazi propaganda from the properties.