Asif Ali Zardari, Bhutto's widower, is also set to deliver a key political speech on Thursday.
Many mourners, who listened to mourning songs and recited verses from the Quran, spent the night in tents outside the Bhutto mausoleum in the rural village of Garhi Khuda Bakhsh.
One mourner, Nabi Bux Kalhoro, said: "We have to win this election - in our leader's words, democracy is the best revenge."
Party officials said they expected tens of thousands of people to turn out to mark Bhutto's "chehlum" - the completion of the mourning period.
Massive turnout
Groups came from across southern Sindh and central Punjab provinces, but others came from the far north of Pakistan Kashmir.
Supporters also gathered in Larkana and in the adjacent village of Naudero, where the Bhutto family ancestral home is located.
The former prime minister was killed in a gun and suicide bomb attack in Rawalpindi on December 27.
The government has accused a Taliban and al-Qaeda linked tribal leader of masterminding her murder, but before her death Bhutto wrote that government and intelligence figures were plotting to kill her.
Ceasefire
Pakistani officials meanwhile said the government will hold talks with the Taliban after the group announced a ceasefire.
A Taliban spokesman in South Waziristan tribal district said a day earlier that Baitullah Mehsud, the key suspect in the murder of Bhutto, had ordered an indefinite truce.
Hamid Nawaz, the interior minister, said: "There is no announced ceasefire, there is a de facto ceasefire between militants and government troops. Both sides are currently holding the fire."
Nawaz said a tribal council, or jirga "comprising representatives of the government and tribal elders will be formed to negotiate peace but I cannot give you any timeframe in this regard".
More than 300 people have died in violence this year, much of it in fighting between Taliban-linked fighters and troops in South Waziristan, the stronghold of Mehsud.