Mohammad Hamid, a police officer, confirmed it was a bomb that tore through the bus.
"The explosive was on the bus," said Javed Iqbal Cheema, an interior ministry spokesman.
Mohammed Afzal, an area police official, said the dead and injured had been transported to different hospitals.
The bus was completely destroyed and mutilated bodies lay on the street, a witness said.
There was no immediate claim of responsibility for either bombing.
Several retaliatory attacks have rocked Pakistan after the government of General Pervez Musharraf crushed an uprising by armed Islamic students at Islamabad's Red Mosque in July.