The 25-year-old was taking part in the first of two planned World Supersport series races, which went ahead despite torrential rain.
Antonelli was involved in a collision with Lorenzo Zanetti on the final corner of the opening lap of the race. He was rushed to hospitaly but later died from his injuries.
The race was immediately called off, and the scheduled World Superbike race later on Sunday was also cancelled.
Rain had started during the final stages of the first World Superbike race earlier on Sunday, during which several top riders - including pole sitter Davide Guigliano - crashed out.
That race was eventually won by GoldBet BMW’s Marco Melandri with team-mate Chaz Davies in second and Ayrton Badovini third.
Despite the terrible conditions the World Supersport race did start, but was red flagged after the crash involving Pata Honda’s Zanetti and Team Goeleven rider Antonelli.
Yakhnich Yamaha’s Sam Lowes had been leading before the race was stopped, with organisers immediately deciding to call off the remaining races.