Maduro won re-election with 51.2 percent of votes, while opposition candidate Edmundo Gonzalez Urrutia received 44.2 percent, the electoral council announced.
Maduro, 61, addressed supporters at the presidential palace minutes after the announcement to celebrate the declaration from his loyalist electoral authority.
"I can say, before the people of Venezuela and the world, I am Nicolas Maduro Moros, the re-elected president of the Bolivarian Republic of Venezuela," Maduro said.
"There will be peace, stability and justice. Peace and respect for the law."
But Venezuela's opposition coalition insisted it had garnered 70 percent of the vote, rejecting the figures from Maduro's loyalist electoral authority.
"We want to say to all of Venezuela and the world that Venezuela has a new president-elect and it is (candidate) Edmundo Gonzalez Urrutia," opposition leader Maria Corina Machado told journalists, adding: "We won."
Costa Rican President Rodrigo Chaves also denounced the official result as "fraudulent," while Chile's president described it as "hard to believe."
Peru announced it had recalled its ambassador for consultation over the results.
US Secretary of State Antony Blinken expressed "serious concerns" that the result did not reflect the will of Venezuelan voters.
Independent polls had predicted Sunday's vote would bring an end to 25 years of "Chavismo," the populist movement founded by Maduro's socialist predecessor and mentor, the late Hugo Chavez.
Since 2013, Maduro has been at the helm of the once wealthy petro-state where GDP dropped by 80 percent in a decade, pushing more than seven million of its 30 million citizens to emigrate.
He is accused of locking up critics and harassing the opposition in a climate of rising authoritarianism.
Gonzalez Urrutia had replaced popular Machado on the ticket after authorities loyal to Maduro excluded her from the race.
Machado, who campaigned far and wide for her proxy, had urged voters on Sunday to keep "vigil" at their polling stations in the "decisive hours" of counting amid widespread fears of fraud.
Maduro had previously warned of a "bloodbath" if he lost.
Rejecting opinion polls, the government relied on its own numbers to assert Maduro would defeat Gonzalez Urrutia, a little-known 74-year-old former diplomat.
Maduro counts on a loyal electoral apparatus, military leadership and state institutions in a system of well-established political patronage.
On Friday, a Venezuelan NGO said Caracas was holding 305 "political prisoners" and had arrested 135 people with links to the opposition campaign since January.
Gonzalez Urrutia had said the opposition was "prepared to defend" the vote and trusted "our armed forces to respect the decision of our people."
He added there had been a "massive" voter turnout.
Ballots were cast on machines that print paper receipts placed into a container. The electronic votes go directly to a centralized CNE database.
The opposition had deployed about 90,000 volunteer election monitors to polling stations countrywide.
Sunday's election was the product of a mediated deal reached last year between the government and opposition.
The agreement to hold the vote led the United States to temporarily ease sanctions imposed after Maduro's 2018 re-election, which was rejected as a sham by dozens of Western and Latin American countries.
But the sanctions were snapped back after Maduro reneged on agreed conditions.
Washington is keen for a return to stability in Venezuela -- an ally of Cuba, Russia and China that boasts the world's largest oil reserves but severely diminished production capacity.
Economic misery in the South American nation has been a major source of migration pressure on the US southern border.
Most Venezuelans live on just a few dollars a month, with the country's health care and education systems in disrepair and the population enduring biting shortages of electricity and fuel.
The government blames sanctions, but observers also point the finger at corruption and government inefficiency.
Machado said earlier Sunday that if Maduro "grabs power," another "three, four, five million" Venezuelans will likely join the exodus.
"What's at stake here goes beyond our borders, beyond Venezuela," she said.
Concerns over the fairness of the vote were earlier stoked when Caracas blocked several international observers, including four Latin American ex-presidents, at the last minute.
About 21 million Venezuelans were registered as voters, but only an estimated 17 million still in the country were eligible to cast ballots.
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