Islamabad, Jun 18 (IANS): At least four university pupils were killed and more than 50 other people injured Monday when a bomb fixed to a car exploded and hit a bus carrying students in Pakistan's Quetta city, media reports said.

The blast took place around 8.20 a.m. when the bus carrying 52 students from the Balochistan University of Information Technology was hit by the explosion inside a small car parked near the Federal Investigation Authority building on Samungli Road.

Geo News said at least 53 people, including four policemen and four women students, were injured.

A TV channel quoted hospital officials as saying that the injured included nine women and five children.

The blast left a two-foot-deep crater on the ground, witnesses said. A motorbike and a rickshaw near the blast site were destroyed, Xinhua reported.

State-run PTV channel said the injured policemen were passing by the site in a car when the bomb went off.

A bomb disposal squad said around 50 kg of explosives were used in the blast.

Police said it was not clear if the bus was the target.

No group has claimed responsibility for the attack.

The toll may rise as at least 20 people are lying in hospitals in critical condition.

Prime Minister Yousuf Raza Gilani has condemned the attack.

Angry students gathered near the hospitals where the injured were admitted, and staged a protest.