(CNN) -- Denouncing election irregularities, Venezuelan opposition candidate Henrique Capriles Radonski demanded a recount and said early Monday that he will not recognize the country's presidential results "until every vote is counted."
His comments came less than an hour after officials said the man former Venezuelan President Hugo Chavez handpicked to be his successor had won the country's presidential vote.
With 99% of votes counted, Nicolas Maduro won 50.66% of votes, National Electoral Council President Tibisay Lucena said, calling the results "irreversible." Capriles won 49.07% of votes, she said.
Fireworks erupted over Caracas as Maduro spoke to cheering supporters, calling on Venezuelans to respect the results and respond peacefully.
"I have won by nearly 300,000 votes. It is the decision of the people," he said.
The closely watched election was the second time in just over six months that voters in the South American country cast ballots in a presidential election.
Capriles is the former governor of Miranda state. He lost to Chavez in October's presidential vote, coming 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 18.9 million Venezuelans are registered to vote in the presidential election.
More than 68% of the country's registered voters had cast ballots by 3 p.m. Sunday, elections officials said.
Far beyond Venezuela's borders, voters lined up at diplomatic offices around the world.
Venezuela's National Electoral Council says that more than 100,000 Venezuelans are registered to vote abroad at diplomatic offices in 88 countries. There are more than 37,681 Venezuelans registered to vote in the United States, according to government figures.
By midafternoon, thousands had cast ballots in New Orleans. Many said they had traveled by car or bus for more than 12 hours to get there from Florida because Venezuela shut down its consulate in Miami last year.
Beatriz Olavarria, who worked to mobilize voters, said it wasn't easy. While campaigning lasted for months before October's election, this time around, the official campaign period was only a matter of days.
"It was much more difficult because the time has been short, and the budget has been low," she said Sunday, "but the excitement has been enormous."
In addition to representatives from national organizations serving as observers, Venezuela's National Electoral Council says 170 foreigners have been invited to witness the elections in Venezuela. The international group includes delegations from the Union of South American Nations and the Atlanta-based Carter Center.
Authorities detained 43 people Sunday for alleged electoral crimes, Maj. Gen. Wilmer Barrientos told reporters.
But elections officials said Sunday evening that the day had proceeded smoothly without major incidents.
"It has developed with total normalcy, with total calmness," said Sandra Oblitas of the National Electoral Council.
Polls were scheduled to close at 6 p.m. (6:30 p.m. ET), but Barrientos said polling stations where voters remained in line would remain open until the last voter cast a ballot.
As polls were closing on Sunday evening, 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," blaming right-wing political opponents for the hacking.
Both candidates' campaigns called on Venezuelans to await official results calmly earlier Sunday.
But they also appeared to be bracing for a fight, with opposition leaders decrying what they said were election irregularities and Maduro's campaign vowing to defend the election results.
Capriles said in a Twitter post that some officials want to change "the desire expressed by the people."
Jorge Rodriguez, the head of Maduro's campaign, stressed that the election had been fair and called on Maduro's supporters to gather in front of the presidential palace.
"We will recognize the results, whatever they are," he said. "And in this we say, that in the scenario that the anti-Chavista candidate wins by one vote, we will recognize it, but in the scenario that we win by at least one vote, we are going to defend this victory with the people of Venezuela."
CNN's Fernando del Rincon, Patricia Janiot and Paula Newton contributed to this report from Caracas. CNN's Gustavo Valdes contributed to this report from New Orleans and CNN's Claudia Dominguez contributed from Atlanta.