Marc Marquez, who started 13th on the grid on his Gresini Ducati, finished second with a third Spaniard, Maverick Vinales, on an Aprilia, rounding out the podium.
Italian Enea Bastianini was fourth on a factory Ducati to climb to second in the championship standings. He trails Martin by 28 points.
Martin, suffering no after-effects of a crash on his Pramac Ducati at the end of qualifying, pulled smoothly away from the field as rivals suffered problems.
The closest challenger in the standings, Francesco Bagnaia, who also crashed late in the morning, had problems with his factory Ducati.
He started second on the grid but was down to 15th on the first lap. The reigning double world champion had slipped to the back by the time he veered off the course of lap four.
Aleix Espargaro started fast from the second row on his Aprilia. Too fast, the stewards ruled, penalising the Spaniard with two long laps. He still managed to take fifth.
Italian Marco Bezzecchi crashed out on lap 10 while running comfortably second on a Ducati VR46.
"The objective was to pull away," said Martin at the finish. "Marco was making it difficult. When he crashed, I just tried to relax and make it to the finish."
Martin crossed the line 2.28 seconds ahead of Marquez, who had surged through the field from the fifth. Marquez said he had surprised himself.
"The target was top seven, top eight," he said. "The pace was there. Super happy. Impressive. But tomorrow will be difficult to repeat the same start which was perfect."
The riders will start from the same grid positions in the main race on Sunday when rain is forecast.
