Ali Haider, 27, was campaigning in Mati area of Multan, 350 km from Lahore, when the gunmen intercepted his SUV and fired indiscriminately.
The gunmen, who came to the spot on motorcycles and a car, took Ali Haider away with them, said Iftikhar Baloch, a close associate of the former premier.
Mohiyuddin Bhutta, the personal secretary of Ali Haider, was killed and four bodyguards were injured, Baloch told PTI.
No group claimed responsibility for the abduction.
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Local police chief Ghulam Mehmood Dogar described the incident as an act of terrorism.
He said police had sealed the district and offered a reward to anyone who provides information about the kidnappers.
Dogar did not rule out the involvement of banned militant groups in the kidnapping.
"At this stage we cannot say anything. Definitely this is an act of terrorism and this is the work of terrorists," he told reporters.
A witness told Geo News channel that the gunmen came to the spot in a car and two motorcycles and indiscriminately fired at Ali Haider's supporters.
The gunmen kept firing as they sped away, the witness said.
The witness said Ali Haider's clothes were stained with blood but he was not sure whether he was wounded .
The abductors were wearing shalwar-kameez and one of them had a beard, the witness said.
Ali Haider, who was campaigning for himself and his elder brother Abdul Qadir, is one of a set of triplets.
Gilani, who has four sons and a daughter, is one of the top leaders of the Pakistan People's Party, which led the outgoing government.