The Finn walked away after his car suddenly veered off the track before turn eight, one of the quickest points on the fastest circuit on the calendar where cars reach speeds of around 330kph.
It smashed at an angle into a concrete wall, narrowly missing two flag marshals as it skimmed uncontrollably past them before plunging into the tyre barriers.
Television images showed Raikkonen vainly trying to steer the unresponsive car away from the wall before the impact, which showered him in debris.
He left the car unaided and clambered over the barrier to safety before returning to the Ferrari garage.
The session was stopped for 20 minutes to clear the track, with McLaren's championship leader Lewis Hamilton top of the timesheets ahead of double world champion Fernando Alonso.
Raikkonen, preparing for his first race in Italy as a Ferrari driver after replacing retired seven times champion Michael Schumacher at the end of last season, had been ninth fastest.
The Finn is fourth in the championship and 16 points adrift of Hamilton, who leads Alonso by five. Ferrari's Brazilian Felipe Massa is third and a point ahead of Raikkonen.
