Islamabad, Pakistan (CNN) -- A bomb killed 15 people and injured 48 others in southwestern Pakistan on Thursday, a senior police official said.
The bomb was planted on a bicycle outside a madrassa, or Muslim religious school, in the city of Quetta, said Qazi Abdul Wahid, a senior police official in the city.
It exploded as a ceremony was under way at the school, killing at least four students and seven people who were passing by, he said. Five children were among the dead, Wahid said.
Prime Minister Syed Yusuf Raza Gilani "expressed deep sorrow and grief over the loss of innocent lives" and urged authorities to investigate the bombing and treat the injured.
CNN's Aliza Kassim contributed to this report.