ISLAMABAD -- Police said 41 people were killed when unknown gunmen opened fire at a passenger bus in Pakistan's southern port city of Karachi on Wednesday morning, and many others sustained injuries.

Local Dunya TV reported that at least six to eight people coming on motorbikes opened fire at the bus in Safoora Chorangi area of Karachi, the capital city of the country's south Sindh province, and fled the scene.

Eyewitnesses said that the attackers surrounded the bus after stopping it and opened fire at it.

The injured people are being shifted to two different hospitals in the city and many of them are said to be in critical condition.

Police reached the attack site and investigations are being carried out.

No group has claimed responsibility for the attack yet.