Hundreds of thousands of people filled the streets of Karachi after Bhutto, who leads the Pakistan People's Party (PPP), flew in from Dubai. She is expected to bid for a third premiership, having been out of power for 11 years.
"I'm going back for the people of Pakistan and it is my faith that everything will be all right," she had said before leaving.
Extra police forces and bomb disposal experts have been redeployed from other parts of the country to provide protection against threatened suicide bomb attacks by militants.
Bhutto pledged a day before her return to Pakistan to put an end to extremism and to develop democracy in the country if she returns to power. Speaking at a news conference in Dubai, she confirmed her intention to run in the parliamentary elections slated for January next year.
The 54-year-old rejected President Gen. Pervez Musharraf's request that she delay her return to the country.
She said had received threats from militants, but that: "Muslims know that if they attack a woman they will burn in hell."
Bhutto served as prime minister of Pakistan on two separate occasions, but both her governments were brought down amid corruption allegations.
President Musharraf has declared an amnesty for Bhutto to protect her against the corruption charges, but the Pakistani Supreme Court is set to hear appeals against the amnesty.
Bhutto has she said believes the court will leave the amnesty in place, and has dismissed the allegations as politically motivated.
