ISLAMABAD, Pakistan — Tensions were running high on Saturday in southwestern Pakistan after dozens of heavily armed gunmen, wearing the uniforms of security forces, stopped two buses, singled out ethnic Pashtuns and fatally shot at least 22 of them on Friday night.

Hundreds of protesters were holding a sit-in outside the Governor’s House in Quetta, the provincial capital of restive Baluchistan Province, demanding punishment for the killers. Most roads in the city were deserted, and businesses were closed to protest the killings.

No group has claimed responsibility for the attack, but similar attacks have been carried out by Baluch separatists, who since the mid-2000s have waged a low-level insurgency in the oil- and mineral-rich province. The insurgency is an outgrowth of past calls for greater autonomy and a greater share of the natural resources there.

The attack took place in Mastung, about 40 miles southeast of Quetta. The buses had been on their way to Karachi, the southern port city in neighboring Sindh Province.