More than 200,000 people gathered at the Bhutto family mausoleum in Garhi Khuda Bakhsh in the southern province of Sindh to pay their respects to Benazir and to hear Bilawal Bhutto Zardari make his first major public speech.
Bhutto, twice elected prime minister, was killed in a gun and suicide attack after an election rally in Rawalpindi, the headquarters of Pakistan's army, on December 27, 2007. No one has ever been convicted of her murder.
In an impassioned speech amid tight security, Bilawal -- the third generation of his family to go into politics -- promised to continue the fight for the poor and against "anti-democratic forces".
"Bhutto is an emotion, a love," he said.
"Every challenge is soaked in blood, but you will be the loser. How ever many Bhuttos you kill, more Bhuttos will emerge from every house."
The Bhutto family has been a force in Pakistani politics for nearly all of the country's 65-year history.
Benazir's father Zulfikar Ali Bhutto, who founded the PPP, led the country from 1971 until he was ousted in a military coup in 1977. He was hanged in 1979 after being convicted of authorising the murder of a political opponent.
The Bhuttos are an almost ever-present element in the rhetoric of PPP leaders, who frequently eulogise the party's two "martyrs" as champions of the common man's struggle against a repressive "establishment".
Security for Oxford-educated Bilawal's speech was tight -- surveillance helicopters hovered overhead and police said more than 15,000 officers had been deployed, as well as some 500 government paramilitary forces.
At Benazir's grave, women beat their chests and wept as they touched the tomb as a mark of respect and shouted "Long Live Bhutto" and "Bhutto was alive yesterday, Bhutto is alive today".
A general election is due in the spring and though the 24-year-old will be too young to stand -- the lower age limit is 25 -- he could act as a figurehead for the campaign.
President Zardari, who came to power in elections held a month after his wife's murder, is barred from leading the PPP election campaign. He is also hugely unpopular, tainted by years of corruption allegations.
Bilawal also took aim at Pakistan's vocal Chief Justice Iftikhar Muhammad Chaudhry, demanding to know why the Supreme Court could find time to deal with issues such as compressed gas and sugar prices but not punish his mother's killers.
"I asked the top judge, can't you see the blood of Benazir Bhutto on the roads of Rawalpindi?" Bilawal said.
"I, as an heir of Bhutto, ask why the killers of my mother have not been punished, while you have time to hold the trial of my mother's grave."
The ruling Pakistan People's Party (PPP) and the judiciary have been at loggerheads for more than two years over the Supreme Court's attempt to reopen graft cases against Benazir and her husband President Asif Ali Zardari -- Bilawal's father.
The PPP regards the efforts as tantamount to putting the dead former prime minister on trial.
Bilawal, who has been co-chairman of the PPP with his father since Benazir's death, in May accused former military ruler Pervez Musharraf of "murdering" his mother by deliberately sabotaging her security.
A UN report in 2010 also said the murder could have been prevented and accused Musharraf's government of failing to properly protect Bhutto.
The Musharraf regime blamed the assassination on Pakistani Taliban chief Baitullah Mehsud, who denied any involvement and was killed in a US drone attack in August 2009.