Bhutto, 54, died in hospital in Rawalpindi. Ary-One Television said she had been shot in the head.
Police said a suicide bomber fired shots at Bhutto as she was leaving the rally venue in a park before blowing himself up.
"The man first fired at Bhutto's vehicle. She ducked and then he blew himself up," said police officer Mohammad Shahid.
Police said 16 people had been killed in the blast.