BARCELONA (Dispatches) - Lorenzo, who had charged from 10th on the grid to join the lead battle, fell at the sharp Turn 10 left-hander while trying to pass Maverick Vinales, taking down his compatriot, as well as Dovizioso and Valentino Rossi in the process.
Marquez overtook pole-sitter Fabio Quartararo off the line, but yielded to a fast-starting Dovizioso at Turn 1, and was soon also briefly overtaken by Vinales.
Having regained second place, the championship leader pressured Dovizioso, and snuck past the Ducati man on the second lap at Turn 10.
At that same corner, Lorenzo committed down the inside of Vinales for third place but hit the deck on corner entry.
His bike clattered into Dovizioso's rear, and the resulting crash removed both Vinales and Rossi – who had gone wide to avoid the pile-up - from the race.
The crash left Marquez on his own up front, and promoted Ducati's Petrucci to second place, the Italian unable to keep pace with the Honda man but managing to fend off both Suzuki's Alex Rins and Quartararo, who had been as low as ninth after the opening lap.
On the 10th lap, Rins launched his bike down the inside of Petrucci at Turn 1, briefly getting ahead but running wide on exit, allowing Petrucci to sweep around the outside of Turn 2.
The same sequence of events was repeated two laps later, and it was another two laps before before Petrucci opened the door at the sweeping Turn 4 right-hander and allowed Rins to barge past.
But Petrucci then slipstreamed back ahead of Rins at Turn 1 on lap 17, and when the Spaniard tried another Turn 1 move next time by, he came in too hot, just avoided collecting Petrucci and nearly highsided off his GSX-RR, tumbling down the order.
Thus had unsettled Petrucci enough to allow Quartararo to get past just a few corners later with a move at Turn 4, but by that point Marquez was already more than five seconds clear.
*****Andrea Dovizioso led after a quick start but Marquez (93) claimed the win