UPDATE 1: Marc Marquez "deemed to have escaped any serious injury".
UPDATE 2: HRC state that Marquez "seems ok, just heavy swelling to the chin area after the serious impact".
MotoGP rookie and title contender Marc Marquez has been taken to the Mugello medical centre after falling along the undulating 200mph main straight during this afternoon's second practice session in Italy.
The Austin winner, fastest in this morning's damp practice, got out of shape as he touched the brakes in preparation for turn one.
The Spaniard veered to the left, putting him onto the grass at the side of the circuit and causing the fast fall from his Repsol Honda.
Mugello has the highest MotoGP top speeds of the season, with Marquez's team-mate Dani Pedrosa
setting the all time record of 217mph (349km/h) in 2009. Marquez set the fastest speed of the day at 213mph (342km/h) just before his accident.
Marquez - who was fortunate to avoid contact with the unprotected concrete trackside wall as he and his RCV skimmed to a halt - was conscious and moving after the incident, but clearly required medical checks and was carried to the ambulance on a stretcher.
Second in the championship Marquez, the reigning Moto2 champion, is the only rider to have finished on the podium in all four races so far this year.
Fellow MotoGP rookie Bradley Smith suffered a highside from his Tech 3 Yamaha shortly after Marquez's fall, which occurred midway through the dry 45-minute session, and is undergoing examination for a "suspected left arm injury".