The United States, Mexico, Colombia, Peru and other nations said they did not recognise the results of the election of a new "Constituent Assembly" superseding Venezuela's legislative body, the opposition-controlled National Assembly.
The European Union expressed "preoccupation for the fate of democracy in Venezuela" and said it, too, doubted it could accept the results.
However, Russia, Cuba, Nicaragua and Bolivia have stood by Maduro, who shrugged off mass protests and US sanctions on some officials to see through the election.
"It is the biggest vote the revolution has ever scored in its 18-year history," Maduro said, dressed in the red associated with the socialist revolution started by his late mentor, Hugo Chavez.
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However the leader of the opposition congress, Julio Borges, said Venezuela "is becoming more divided and isolated in the world."
According to the opposition, voter turnout was closer to 12 percent, a figure more aligned with the lack of lines that were seen at many polling stations.
According to polling firm Datanalisis, more than 70 percent of Venezuelans opposed the new assembly.
Demonstrators were ignoring a ban on protests put in place by Maduro that threatened up to 10 years in prison for violators.
Ten people died in violence surrounding Sunday's election, which saw security forces firing tear gas and, in some cases, live ammunition to put down protests. Among them were two teens and a Venezuelan soldier.
In eastern Caracas, seven police were wounded when an improvised explosive targeted their motorcycle convoy.
With the opposition boycotting Sunday's election, and state employees pressured to vote, the Constituent Assembly is made up solely of members of Maduro's ruling Socialist Party.
It is due to be installed on Wednesday. Its members include Maduro's wife, Cilia Flores.
"We do not recognise this fraudulent process," said an opposition leader, Henrique Capriles. The opposition coalition, the Democratic Unity Roundtable, says the new assembly is a rubber-stamp mechanism for Maduro to rule as a "dictator."
The US envoy to the United Nations, Nikki Haley, called the election a "sham," and the US State Department warned of further "strong and swift" sanctions on Maduro's government.
Russia, however, threw its weight behind Maduro and the election, backing the government turnout figure.
Russia's foreign ministry said in a statement that it hopes countries "who apparently want to increase economic pressure on Caracas will display restraint and abandon their destructive plans."
Bolivia echoed that, urging the world "to respect the democratic process that took place in Venezuela."
Analysts, though, agreed that Maduro's move had swept away democracy in Venezuela.
Eduardo Rios Ludena, a Venezuela specialist at the Paris Institute of Political Studies, said Maduro had "sacrificed democracy" by exaggerating the number of voters who took part.
"In the short term, the Constituent Assembly gives a bit of breathing space to the government" but grave economic consequences would follow, aggravated by further sanctions, he said.
Those would worsen the crisis faced by Venezuela's 30 million citizens, who are suffering through shortages of basic goods -- but would also destabilize the government, which is frenziedly printing money and is reliant on oil exports.
"However it is unclear what they can negotiate on since the opposition wants to remove Maduro from power -- through a fair election process -- and Maduro will do whatever it takes to stay in power," he said.