HO CHI MINH CITY—McDonald's Corp. opened its first restaurant in Vietnam to huge crowds eager to try a Big Mac, with people waiting in long, velvet-roped lines or on motor scooters inching up to the drive-through.

McDonald's let in its first customers on Saturday at its flagship 24-hour store on crowded Dien Bien Phu street in Ho Chi Minh City, the country's economic hub. The crowds frequently outnumbered the available 350 seats, with the largely young clientele snapping selfies with the Golden Arches symbol behind them....