"He is involved in the blasts in Karsaz. Therefore he has been arrested," Nawaz told the AP news agency, referring to the Karachi neighbourhood where the October bombing occurred.
Denial
Hashmat Habib, Akhtar's lawyer, denies he was involved.
Authorities also arrested three other men identified by Akhtar as his sons.
The government of Pervez Musharraf, Pakistan's president, has blamed both the Karachi bombing and Bhutto's assassination on Baitullah Mehsud, the leader of a tribal armed group.
Mehsud has denied the claims, while Bhutto herself disputed his involvement.
In a book she wrote shortly before her death, she claimed that Akhtar was involved in the bombing. She also accused political allies of Musharraf of plotting to kill her.
Akhtar's lawyer said he would challenge his client's arrest in the supreme court.
He confirmed that Akhtar had once fought in Afghanistan and the disputed Himalayan territory of Kashmir but said he had abandoned his involvement with armed groups in 1994 and had no role in the Karachi attack.