Julian Simon has taken his second grand prix victory, and first since Donington Park in 2005, after dominating a wet French Grand Prix at Le Mans.
Rain began falling as the field lined up on the grid, at least ensuring that riders and bikes were prepared for the conditions ahead.
Nicolas Terol shot into an early lead, with Simon the only rider capable of keeping pace as the pair pulled quickly away from the pack, led by Joan Olive and Marc Marquez.
Simon - who crashed out while trying to chase down Aspar Aprilia team-mate Bradley Smith in the early stages of the previous Spanish Grand Prix, handing the Englishman a huge lead - found himself in a near identical situation when Terol fell at the Dunlop chicane on lap 4 of 24.
Simon's advantage then leapt to a distant 19 seconds when second and third place riders Olive and Marquez fell at the final turn of lap 6. Olive lost control of his Derbi, leaving debut pole sitter Marquez with nowhere to go but into the Spaniard.
Smith, who began the race just one point behind Andrea Iannone, was thus promoted to second position, but was soon under pressure from a frantic group of riders disputing third.
That pack was reduced in size as riders crashed out, but Smith was swamped just before the halfway stage, leaving Simon a massive 24 seconds ahead of ISPA Aprilia rookie Jonas Folger, with Stefan Bradl holding fourth in front of Smith and third Aspar rider Sergio Gadea.
German Bradl was next to make an unplanned exit, leaving young 15-year-old countryman Folger - competing in his first full grand prix season - in a safe second place.
Simon remained wheel perfect to the finish, where he took victory and the world championship lead by 27.084sec over Folger, who claimed the podium in only his tenth grand prix. Folger was 3.8sec clear of Gadea, who took the final podium place from Smith with three laps to go.