The BBC's Mike Wooldridge: "One of his aides, his secretary, was killed". Photo: BBC

Mr 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.

Taliban threats

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.

 

Musa Gilani: "If we don't get my brother by this evening, I will not let the election happen"

"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.

Sharif's pledge

The run-up to the 11 May elections has been marred by a series of attacks across the country in which more than 100 people have been killed.

The Pakistani Taliban have threatened to prevent the PPP, the Awami National Party (ANP) as well the MQM party, from conducting their election campaigns because they are considered by the militants to be too secular.

The military has pledged to deploy tens of thousands of troops to polling stations on Saturday to prevent further attacks.

In a separate development, Nawaz Sharif - the man tipped to be Pakistan's next prime minister - promised to end the country's involvement in the US-led war on terror if elected.

Mr Sharif - who leads the Pakistan Muslim League-N (PML-N) - told the BBC the move was necessary for there to be peace in Pakistan and elsewhere in the world.

Pakistan has been part of the US-led fight against Islamist militancy in the region since the 11 September attacks in the US in 2001.

Mr Sharif's remarks may cause concern among Western leaders, the BBC's Orla Guerin reports from Islamabad.

However, Mr Sharif - who served as prime minister twice in the 1990s - declined to say whether he would stop military operations against the Taliban and al-Qaeda.

Meanwhile, Imran Khan - another leading Pakistani politician - is continuing to recover in hospital after falling off a makeshift lift at an election rally earlier this week.

Doctors say that the former cricketer who leads the Tehreek-e-Insaf (Movement for Justice) party - received stitches in the head and treatment for injuries to his spine.