(CNN) -- Election authorities proclaimed Hugo Chavez's handpicked successor Venezuela's president-elect Monday, despite his challenger's demand for a recount.
"It was a result that was truly fair, constitutional and popular," Nicolas Maduro said, criticizing his opponent's refusal to concede.
Maduro secured 50.8% of votes in Sunday's election, while opposition candidate Henrique Capriles Radonski won 49.0%, Venezuela's National Electoral Council said Monday.
The South American country's top election official certified the results at a ceremony in Caracas, saying Venezuela's voting system had worked perfectly.
Maduro, who billed himself throughout the campaign as Chavez's political heir, told supporters Monday that the former president's son-in-law would be executive vice president during his six-year term.
Jorge Arreaza has been Venezuela's science and technology minister and is married to Chavez's daughter, Rosa Virginia.
Earlier Monday, Capriles called on his supporters to protest and slammed Maduro as an "illegitimate" leader.
"If both sides have said that they want to count vote for vote, what is the rush? What are they hiding? Why do we have to accelerate the process?" he said. "What they want is for the truth not to be known."
Many of the opposition candidate's backers took to the streets Monday night, banging pots and pans to protest the government's refusal to recount votes. Capriles called on them to head to local election offices Tuesday.
The head of Maduro's campaign accused Capriles of inciting violence.
"This man, Capriles, does not know how to lose," Jorge Rodriguez, the head of Maduro's campaign, told reporters Monday.
Capriles' repeated demands for a recount left key questions about Venezuela's future unanswered Monday: Will Maduro's supporters stick behind him? Will tension in the deeply divided country boil over after the tight race? And will world leaders recognize the results?
The presidents of Argentina, Bolivia and Cuba were among the leaders who offered congratulations to Maduro on Monday. But the head of a prominent regional body said he supported a recount. And a White House spokesman pushed for an audit of the results.
"The result as reported is extremely close," White House spokesman Jay Carney told reporters. "The opposition candidate and at least one member of the electoral council have called for an audit, which ... in our view, seems like an important and prudent step to take."
Organization of American States Secretary-General Jose Miguel Insulza offered the support of the regional body's election experts for a recount "in the context of deep division and political polarization."
Both Capriles and Maduro have urged supporters to remain peaceful but appeared to be bracing for an intense political fight.
Tension mounts after tight race
Police and National Guard troops stood by, armed with tear gas, as protesters gathered in one Caracas neighborhood Monday night, witness Alejandro Astorman said.
On state television, Maduro vowed to use an "iron fist" against any coup attempts and accused political opponents of trying to destabilize the country.
On Sunday, Maduro said he would have nothing to hide if votes were recounted. On Monday, he said officials had already ruled that the results showing his victory were clear.
"It is the majority," he said. "There is a majority that won, and there is a minority that we recognize and respect, even though many of them hate us."
Capriles stressed that Maduro must also recognize the political will of the millions of voters who supported the opposition.
"Here there is no majority," he said. "There are two halves."
Sunday's closely watched election comes at a time of political polarization and uncertainty for Venezuela. It was the second time in just over six months that voters in the South American country cast ballots in a presidential vote.
'I am not Chavez, but I am his son'
Maduro, 50, has been Venezuela's interim leader since Chavez's death. When he registered to run for the presidency last month, he told supporters, "I am not Chavez, but I am his son."
They weren't blood relations, but in one of his last public appearances, Chavez tapped Maduro as his replacement.
"My firm opinion, as clear as the full moon -- irrevocable, absolute, total -- is ... that you elect Nicolas Maduro as president," Chavez said in December, waving a copy of the Venezuelan Constitution as he spoke. "I ask this of you from my heart. He is one of the young leaders with the greatest ability to continue, if I cannot."
It was the first time Chavez had named a successor.
His comments dramatically changed the political landscape and became the basis for Maduro's push to ascend to the presidency after his mentor's death.
Throughout the campaign, Maduro pledged to continue Chavez's efforts to build "21st century socialism" and said his platform consisted of following the former president's plan for the country.
At rallies for Maduro, a recording of Chavez's voice belting out the national anthem boomed through loudspeakers.
At one campaign event, he told supporters that Chavez appeared to him in the form of a little bird to give him spiritual support.
And Maduro's official campaign theme song began with a militant drumbeat and Chavez's voice, endorsing his candidacy.
Campaigns bracing for a battle
Capriles, 40, said he was pushing a more moderate approach, promising to continue social programs and improve the country's economy.
The opposition candidate lost to Chavez in October's presidential vote, but he came within 10 percentage points of the longtime leader. It was a significant gap, but the closest any opposition candidate ever came to defeating Chavez during his rule.
More than 78% of the 18.9 million Venezuelans registered voted in Sunday's presidential election, Lucena said.
In Venezuela, elections officials said Sunday evening that the day had proceeded smoothly without major incidents.
Authorities detained 43 people for alleged electoral crimes, Maj. Gen. Wilmer Barrientos told reporters.
As polls were closing Sunday, Twitter accounts for Maduro and his party were apparently hacked with posts denouncing "electoral fraud." A group calling itself Lulz Security Peru claimed responsibility, while officials from Maduro's campaign criticized what they said were "dirty tactics." They blamed right-wing political opponents for the hacking.
CNN's Fernando del Rincon, Patricia Janiot and Paula Newton, and journalist Osmary Hernandez contributed to this report from Caracas. CNN's Rafael Romo, Marysabel Huston-Crespo and Claudia Dominguez contributed from Atlanta.