Haider was participating in a campaign meeting at Multan city this afternoon when the gunmen opened fire at his supporters.
His secretary Mohiyuddin and bodyguard were wounded and later succumbed to their injuries, TV news channels reported.
Five others were injured in the attack, news channels reported.
There was no official word on the development.
No group claimed responsibility for the attack or the abduction.
Footage on television showed Ali Moosa Gilani, another of the former premier's sons, crying and trying to control irate workers of the Pakistan People's Party.
A witness told Geo News channel that the gunmen came to the spot in a car and fired indiscriminately at Haider Gilani's supporters.
He said he had seen the gunmen bundle Haider into a car and drive away. The gunmen kept firing as they drove away, the witness said.
The banned Tehrik-e-Taliban Pakistan has warned it will target secular parties like the Pakistan People's Party and Awami National Party ahead of the May 11 general election.
The threats have forced the parties to virtually stop campaigning for the polls.