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.