Already head of the main ruling party, Zardari becomes one of the most powerful civilian leaders in Pakistan's turbulent 61-year history. Last month, he marshaled a coalition that forced stalwart U.S. ally Pervez Musharraf to quit as head of state.
However, he begins with limited goodwill among a population who recall his nickname, Mr. Ten Percent, for alleged corruption during Bhutto's two terms in office as prime minister and doubt his political vision and leadership skills.
He is also untested on the international stage, where he must deal with mounting Western concern over how Taliban and al-Qaida militants have nested in the tribal belt along the Afghan border. Secretary of State Condoleezza Rice said she was looking forward to working with Zardari.
"I've been impressed by some of the things he has said about the challenges that Pakistan faces, about the centrality of fighting terrorism, about the fact that the terrorism fight is Pakistan's fight and also his very strong words of friendship and alliance with the United States," Rice told reporters on a trip to North Africa.
Zardari made no mention of those topics as he savored his triumph over Musharraf, during whose reign he sat for years in jail on graft charges that never produced a conviction.
A beaming Zardari hugged and shook hands with supporters and well-wishers gathering for a dinner Saturday in the gardens of the prime minister's residence on a hill overlooking the capital. In a brief speech, he rejected criticism that he would be a divisive leader and took a swipe at Musharraf.
"To those who would say that the People's Party or the presidency would be controversial under our guardianship, under our stewardship, I would say listen to democracy," he said.
Echoing one of Bhutto's favorite slogans, he called democracy "the best revenge" against military rulers.
Zardari has surprised many with his ability to concentrate power since his wife was assassinated in a December gun-and-bomb attack blamed on Taliban militants and he inherited her party's leadership.
Former Prime Minister Nawaz Sharif's party abandoned the coalition and switched to the opposition last month. But Zardari quickly won support from smaller parties, suggesting he could provide some stability as the country faces soaring inflation, power shortages and widening trade and budget deficits.
Official results gave Zardari more than two-thirds of the votes - two opponents sharing the rest - setting the stage for him to be sworn in within days.
Pro-Zardari lawmakers, some in tears, shouted "Long live Bhutto!" as the vote tallies came in. The couple's two jubilant but tearful daughters, one carrying a portrait of their late mother, smiled and hugged friends in the gallery of the National Assembly.
In the southern city of Karachi, capital of Zardari's home province, supporters waved his party's tricolor flags, beat drums and danced in the streets, chanting "Zardari is the strongest." There was nothing festive about the mood in Peshawar, the main city of the Taliban-plagued northwest, which suffered the latest in a string of deadly suicide attacks.
Officials and witnesses said a pickup truck packed with explosives demolished a security checkpoint on the edge of the city Saturday, killing at least 30 people, including five police officers, and injuring dozens more.
No one immediately claimed responsibility. In recent weeks, however, the Pakistani Taliban have said they carried out a string of suicide bombings they called revenge for military offensives in the northwest region, which borders Afghanistan.
In more violence reported Saturday, 15 civilians and nine militants died when residents of the restive Swat valley foiled an attempt by insurgents to kidnap a pro-government elder, and then were attacked.
Pakistan has struggled to contain rising militancy in its borders, and the fledgling government has tried both peace talks and military operations.
It's an effort watched closely in the West, where officials worry that militants have safe havens in the northwest from which they plan attacks in neighboring Afghanistan - and could hatch another 9/11-style plot against North America or Europe.
A recent U.S.-led ground assault across the border on a Pakistani tribal region, said to have killed at least 15 people, prompted protests from the government and suggested that American patience with Islamabad is wearing thin.
Like his late wife, Zardari is generally considered a pro-West liberal. He is not expected to change Musharraf's commitment to the U.S. war on terrorism, insisting the battle against militants is Pakistan's own war. But a key test will be how much clout Zardari wields over the powerful military.
As president, Zardari will have the power to dissolve Parliament and appoint army chiefs, and chairs the joint civilian-military committee that controls Pakistan's nuclear weapons.
He has said he will relinquish some of the power accumulated by Musharraf. However, Zardari has not made clear how far he will go, sustaining concern that one would-be strongman is replacing another.
Friends say Zardari remains underestimated, even after he nimbly stepped into Bhutto's political shoes and outmaneuvered both Sharif and Musharraf.
"I've seen a changed person after his wife's death. I've found him to be an extremely astute politician, which was perhaps overshadowed by the towering personality of his wife," said Wajid Hasan, Pakistan's High Commissioner in London.
Many ordinary Pakistanis also remain to be convinced. "We want him to make parliament sovereign and to evolve a clear policy on the war on terror" as well as tackle crippling inflation, said Muhammad Azam, a 33-year-old bank employee from Lahore. "I want to convey to Zardari that he is not a leader by choice, but by chance. Now he has to prove his worth."
Associated Press writers Nahal Toosi and Asif Shahzad in Islamabad, Riaz Khan in Peshawar, Ashraf Khan in Karachi, Babar Dogar in Lahore, Paisley Dodds in London, and Matthew Lee in Algiers contributed to this report.