Two blasts that rocked Karachi early Friday left 140 people dead and over 500 injured as hundreds of thousands of people lined the streets to greet Bhutto's motorcade, who flew in from Dubai after eight years of self-imposed exile.
"We categorically reject the proposal. We are conducting an investigation that is totally objective and unbiased," said Aftab Sherpao.
Bhutto, who was unhurt in the attack, earlier called for British and American investigators to be involved in the probe, saying their assistance would ensure the investigation was unbiased.
She already announced she suspected supporters of former military dictator General Zia-ul-Haq of orchestrating the attack. Zia, who overthrew Bhutto's father Prime Minister Zulfikar Ali Bhutto and had him executed in 1979, died in a car crash in 1988.
Police said the attack, one of the bloodiest in Pakistan's history, was perpetrated by suicide bombers, a severed head believed to belong to one of the bombers has been found by police, and his identity is being established, according to national TV channels.
Bhutto is set to run in the parliamentary elections slated for January next year. She pledged to put an end to extremism, and to develop democracy in the country if she returns to power. She said she had received threats from militants shortly before her arrival.
Bhutto, who leads the center-left Pakistan People's Party (PPP), served as Pakistan's prime minister on two separate occasions, but both her previous governments were brought down amid corruption allegations and she was forced to flee in 1999, dismissing the allegations as politically motivated.
