Pakistan former Prime Minister, Mr Yousuf Raza Gilani told the BBC his son, Ali Haider, a candidate for the Pakistan Peoples’ Party (PPP) was seized in the central city of Multan.
He accused his political opponents of being behind the attack, which came ahead of Saturday’s elections.
One person was reportedly killed when the attackers opened fire at the rally.
No group has so far claimed responsibility for Thursday’s attack.
Eyewitnesses say the gunmen arrived at the gathering in a black Honda car and a motorbike.
“A couple of them started shooting,” a teenager at the rally told Pakistan’s Geo TV.
“A man standing in front of Gilani was hit and fell down. Then they grabbed Gilani, put him in the car and sped away.”
Reports say the person who died in the shooting could have been Ali Haider Gilani’s bodyguard or secretary. Another five people were injured.
Eyewitnesses say a bullet also hit Ali Haider and he was bleeding when the kidnappers put him in the car, Pakistan’s Express Tribune newspaper reports.
Ali Haider, the youngest son of the Ex-Prime Minister is contesting a seat in the Punjab provincial assembly.
“We want our brother back tonight. If we don’t get him, we will not allow elections to be held in our area,” his elder brother Ali Musa, who was in tears later told reporters.
Police have now sealed off all entry and exit point in Multan, and a massive search operation is under way, local media report.
Yousuf Raza Gilani served as prime minister until June 2012, when he was forced out of office by the Supreme Court over his refusal to pursue a corruption case against President Asif Ali Zardari.
But it is still a powerful political family, with Mr Gilani’s sons standing in the elections to the provincial and national assemblies, the BBC’s Mike Wooldridge in Islamabad says.