Police said Bhutto was unhurt, but at least 50 people were reported killed and more than 150 wounded in the attacks in Karachi, according to news agencies. The death toll was expected to rise.
A small blast was followed by a much larger one mere feet from the front of the truck carrying Bhutto. The explosion shattered windows in her vehicle and set a police escort vehicle on fire, reported the Associated Press.
Bhutto was then rushed from the area under a contingency plan.
When Bhutto arrived in the country earlier in the day, she was greeted by crowds of supporters waving flags and cheering. She is expected to seek an unprecedented third term as premier.
"I counted the hours, I counted the minutes and the seconds, just to see this land, to see the grass, to see the sky. I feel so emotionally overwhelmed," a tearful Bhutto told the AP. "And I hope that I can live up to the great expectations which people here have."
Bhutto, 54, left Pakistan amid corruption charges in 1999. A constitutional amendment would be needed for her to seek a third term, something currently not allowed under Pakistani law.
She intends to lead her Pakistan People's Party in national elections expected in early January, part of a plan to return the country to civilian rule.
Army chief Musharraf, who took power in a 1999 coup, is experiencing a weak period politically and is talking with Bhutto about a possible power-sharing agreement.
The United States is believed to have quietly supported their alliance in order to keep nuclear-armed Pakistan pro-Western and committed to fighting al-Qaida, according to Reuters.
Authorities deployed about 20,000 security personnel for Bhutto's arrival to protect her against threatened suicide attacks by militants.
