Bhutto, the son of former prime minister Benazir Bhutto who was assassinated in 2007, also pledged to defend the country's minorities -- increasingly under siege from powerful religious lobbies and often targets of Islamist violence.
"To the Christian and other minority communities in Pakistan, we will defend you," he said at a memorial ceremony in London for Salman Taseer, the governor of Punjab province who was killed by his own security guard last week.
"Those who wish to harm you for a crime you did not commit will have to go through me first."
Taseer, a leading politician in the ruling Pakistan People's Party (PPP), had spoken out in defence of a Christian woman, Aasia Bibi, who had been sentenced to death under blasphemy laws which critics say are misused, often to settle personal scores.
The man who confessed to his killing has been celebrated as a hero by some in Pakistan, and up to 50,000 people joined a rally organised by religious parties in the city of Karachi on Sunday to oppose any changes in the blasphemy laws.
Bhutto condemned all those who justified violence in the name of Islam, through suicide attacks and murder.
"To those who are praising or justifying these crimes, I say: you along with the killers of Shaheed (martyr) Salman Taseer are the real blasphemers."
Taseer's killing has further cowed those in Pakistan promoting liberal, secular views, and PPP politicians including Bhutto's father, President Asif Ali Zardari, have been criticised for failing to speak out.
Bhutto, who is studying in England, said "the dark forces of violent extremism, intolerance and bigotry are intent on devouring our country and our faith" and Taseer's killing was meant to cow its opponents into remaining silent and frightened.
He said he would uphold the legacy of his mother and his grandfather, former prime minister Zulfikar Ali Bhutto, who was hanged in 1979. "I will not be silenced by fear."
He also condemned clerics who warned against mourning Taseer's death, saying of those who threatened Muslims who prayed or grieved for him: "You too shall be defeated."
Five hundred religious scholars from the mainstream Barelvi religious tradition, usually seen as more moderate than Taliban militants who follow Deobandi Islam, had praised Taseer's killer and said those who mourned his death could meet the same fate.
"The assassination of Shaheed Salman Taseer is not about liberals versus conservatives or moderate versus radical Islam. It is about right and wrong," Bhutto said.
Religious parties rarely win much electoral support in Pakistan but they have the capacity to bring large numbers out into the streets. The PPP-led coalition government has been criticised for failing to deliver the kind of governance needed to turn the tide against militancy.