In a report on Rimini Today (read report here) that also contains an image of the car that hit Hayden, it is reported that the American was struck by a Peugeot which ended with the car's windshield being smashed.
Hayden was treated by medical staff at the scene of the road accident before being taken to the local hospital in Rimini where he is being treated for "severe chest and head injuries", according to the report.
It has been reported by motorsport.com that he remains in a serious condition.
"Nicky Hayden (Red Bull Honda World Superbike Team) has been involved in a training incident which saw the American be hit by a car when he was out training on his bicycle," said a statement on worldsbk.com.
"The incident occurred along the Rimini coastline this afternoon, where he has been spending time following the Motul Italian Round at Imola
"Following the incident, emergency services were called to the scene and Hayden was transported to the local hospital. Ambulances arrived and Hayden was taken directly to the hospital.
"More updates will come as soon as they are announced."
The Kentucky rider, 35, is a former MotoGP world champion who rode for Honda and Ducati for 12 years between 2003-2015, and famously beat Valentino Rossi to the MotoGP title in 2006.
Nicky Hayden of USA and Red Bull Honda World Superbike team prepares to start on the grid during the Race 1 during the FIM Superbike World Championship - Race 1 at Enzo and Dino Ferrari Circuit on May 13, 2017 in Imola, Italy.
Image credit: Eurosport
He has switched to World Superbikes over the past two seasons, and won his first World Superbike race at Sepang in Malaysia a year ago.
Michele Scarponi, the 2011 Giro d'Italia cycling champion, was killed last month after being hit by a van while training close to his home in Filottrano in eastern Italy.
Britain's Tour de France champion Chris Froome escaped unhurt this month while having his bike written off in a hit-and-run incident in France.