The attackers, who were said by police to have arrived in a pickup truck, intercepted the bus on the outskirts of the provincial capital of Quetta. Five people were wounded.

"After intercepting the bus, four to five gunmen went in to the bus and opened fire and then fled," senior police Hamid Shakeel said.

The bus was carrying mostly Shi'ite Muslims from the Hazara community, who were returning from Quetta.

One of the wounded men, Hussain, said from the hospital that about 45 minutes outside of Quetta, two men already on the bus ordered the driver to stop and started shooting.

Ten people were killed on the spot while three died at a hospital. There were about 20 people on the bus.

The wounded were driven to a hospital in the brightly painted bus, where protesters enraged by the attack, set it on fire after it was emptied of passengers, according to Geo TV, a private Pakistani television channel.