Bhutto Zardari, who has two younger sisters, was educated in Dubai and London and studied history at Oxford University before returning to Pakistan in 2007 after his mother's death.

His Pakistan People's Party (PPP) ruled the country from 2008 to 2013, but it became tarnished by a series of confrontations with the powerful Supreme Court over corruption scandals.

After people became disillusioned with its image and policies, it was voted out in a landmark election last year that was the first time in Pakistan's short but turbulent history that one elected civilian government replaced another.

Bhutto Zardari has a tough task in reviving his party's fortunes, underscored by a weak result in a by-election in the central city of Multan this month in which the party's candidate came in third, drawing only a few thousand votes. The winner of the election was an independent candidate backed by opposition politician and former cricket star Imran Khan.

The emergence of Bhutto Zardari as an opposition leader comes at an opportune time for the PPP and is likely to be a worry for Sharif. The incumbent's authority has been shaken by weeks of anti-government protests led by Khan and Tahir ul-Qadri, a firebrand cleric.

Khan has vowed to continue his two-month campaign to oust Sharif even as Qadri told supporters last Tuesday to end their protests in the capital.

Qadri told followers that protests would continue elsewhere.

Convincing people he is a force to be reckoned with, however, will be an uphill task for Bhutto Zardari, whose youth may prevent him from being taken seriously beyond the PPP's stronghold in the southern province of Sindh.

"These political orphans and puppets would want us to be a dictatorship again," he said, referring to Khan and Qadri and their protests.

"But Pakistan is over that. We are a democracy. We have had a civilian transfer of power."

Bhutto Zardari, whose age did not allow him to contest the 2008 elections, said he would rely on Pakistan's young population for support and make fighting poverty his central agenda.

"Sixty per cent of the population of Pakistan is young … and of course I, being 26, I think can relate to them more than any other Pakistani political leader can.

"For me, serving the people … is about poverty alleviation."

The Bhuttos have often portrayed themselves as champions of the poor in a country where feudal landlords own vast tracts of land and agricultural workers often live in deep poverty.

As well as his youth, Bhutto Zardari can draw on a name more evocative than any other in Pakistan. His family's story is as torrid as the country's; his mother Benazir was assassinated at an election-campaign rally in 2007, and his grandfather was hanged by a military dictator in 1979.

Benazir remains a powerful symbol and people often refer to her as a martyr. Islamabad's airport and a scheme to give cash to poor families have been named after her.

Last weekend's rally, touted as Bilawal's political debut, marked the seventh anniversary of the bomb attack on Benazir's homecoming parade in Karachi on October 18, 2007. The attack killed 139 people in the deadliest single terror attack on Pakistani soil. She survived but was assassinated the following December.

Given the bloody history, ultra-tight security measures were in place for the rally in Karachi, where tens of thousands of supporters sang and danced, waving the flag of the PPP.

Bhutto Zardari addressed the crowds from the same bullet-and-bomb-proof truck that his mother used for the ill-fated parade. "I start this journey for my people, for the martyrs, for my mother," he wrote on his Twitter page ahead of the rally. "Boarding the truck bought back some painful memories."

While Bhutto Zardari's remarks about the poor are consistent with the PPP's traditional position, he is far more hawkish than his party has been on the issue of Pakistan's long-standing rivalry with India.

The PPP's five years in power were marked by an improvement of ties between the neighbours.

The two countries came close to signing a bilateral trade deal that was called off by the generals after Sharif came to power, underlining the military's long tradition of dictating policy to civilian leaders from behind the scenes.

In recent weeks, armed forces from Pakistan and India have engaged in their worst clashes in decades in the disputed Himalayan region of Kashmir.

"The United Nations Security Council, the people of Pakistan, Pakistan as a country and the people of Kashmir all agree on what the way forward is; it's only India that keeps making excuses and sabotaging peace," Bhutto Zardari said.

"My generation, our generation on both sides of the border won't put up with this," he added, in surprisingly strong comments.

Bhutto Zardari has been an outspoken critic of the hardline Islamist Taliban movement, which threatened his party with attack during the run-up to the 2008 election.

After his mother was killed, her widower, Asif Ali Zardari, returned from self-imposed exile to contest the election successfully. He remains co-chairman of the PPP along with his son.

Bhutto Zardari said the PPP was ultimately responsible for a major military operation against the Taliban launched in the lawless tribal areas in July, and said Sharif, who was in favour of peace talks with the militants, had been reluctant to give the go-ahead.

"It only happened because of the political pressure mounted by the Pakistan People's Party and our fierce, vocal and brave opposition," Bhutto Zardari said.

"The Nawaz Sharif government was reluctant, and the peace process was a consensus built among the right-wing parties that dominate discourse in Pakistan. We took it upon ourselves to go against this policy of appeasement."

Sharif originally pursued negotiations with the Taliban to end the bloodshed, angering the Pakistani military in the process, but violence scuppered the talks.