At least four people were killed and 52 others injured when a roadside planted bomb hit a school bus in Pakistan's southwest city of Quetta on Monday morning, said hospital and police sources.

Local TV channel Express quoted hospital sources as saying that the injured included 38 men, nine women and five kids. Another local state-run TV channel PTV said the injured also included four policemen who happened to pass by the blast site in a vehicle when the bomb went off.

Local media quoted hospital sources as saying over 20 people out of the injured were in critical condition and the death toll may further rise.

According to Xinhua sources in Quetta and local media reports, the blast took place at about 8:20 a.m. local time when a bus carrying at least 52 students from Balochistan IT University was hit by a bomb fixed inside a small Suzuki car parked by the roadside near a Federal Investigation Authority (FIA) building on the Smangli Road of the city.

The blast left a two-foot-deep crater on the ground, said eyewitnesses, adding that several vehicles including a motorbike and a rickshaw nearby were also destroyed.

Security forces and rescue teams rushed to the site shortly after the blast was reported.

The injured have been shifted to different hospitals in the city and an emergency has been declared in the hospitals of the city.

Bomb disposal squad officials said that an estimated 50 kg of explosives were used in the blast.

No group has claimed responsibility for the attack.

Rehman Malik, a top official in charge of Home Ministry in the country, condemned the attack and said that terrorists in Balochistan province, of which Quetta is the capital, were playing in the hands of their foreign masters.

Pakistani Prime Minister Yusuf Raza Gilani also condemned the attack and directed the hospital authorities to provide the best possible medical treatment to the injured people.

Angry students staged a protest nearby the hospitals where the injured were admitted following the blast.