The only son of Benazir Bhutto, Pakistan's assassinated former prime minister, has told tens of thousands of supporters that he would fight for his party's revival in an appearance intended to mark the official launch of his political career.

"The fountainhead of our power is the people," Bilawal Bhutto Zardari, 26, told the crowds in the port city of Karachi, Pakistan's financial hub.

Bhutto is tipped to become, main opposition group, the Pakistan People Party's (PPP) next leader.

Police said 150,000 people gathered to hear the young leader speak. Security was tight during the rally, where crowds of supporters sang and danced, waving party flags.

"If you want to save Pakistan, the only answer is Bhuttoism and the PPP," Bhutto said.

In his two-hour speech, delivered in Urdu, Bilawal condemned the homegrown Pakistani Taliban and other similar groups including the Islamic State of Iraq and the Levant, which controls large swathes of Iraq and Syria.

He expressed fears his country could breed its own sectarian war, telling the crowd: "One fine day another Osama bin Laden would emerge... and some puppet would announce the opening of a Taliban office in Peshawar, which would ultimately become an Islamic State [of Iraq and the Levant] office."

Groomed to lead PPP

Bilawal is being groomed to lead the PPP by his father Asif Ali Zardari, who was Pakistan's president from 2008 until last year.

In the rally, symbolically, he stood on the roof of the same bus where his mother was assassinated exactly seven years ago in a gun and bomb attack after holding an election rally in the city of Rawalpindi in 2007. At least 180 people were killed that day.

The Bhutto dynasty has had a turbulent history, reflecting Pakistan's own rises and falls in past decades. Bilawal's grandfather, the founder of the PPP, was hanged by a former military ruler in 1979.

The PPP ruled Pakistan from 2008 to 2013 until it was voted out in a landmark election that marked the first time in Pakistan's military coup-prone history one elected civilian government replaced another.