The United States condemned Venezuela’s election as “an insult to democracy” and opposition leaders rejected a “fraud foretold” on Sunday as low voter turnout looked to have spoiled what President Nicolás Maduro had promised would be a great “fiesta” of democracy.

Venezuela’s 34,000 polling stations – open since 6am when Maduro cast the first vote – had been due to close at 6pm. But as darkness fell, state broadcaster Telesur announced they would stay open “as long as there are people in line to cast their vote”.

In a tweet posted shortly before polling stations should have closed, the US mission to the United Nations indicated it would reject the result, although there was no indication from Venezuelan authorities when one might be announced. “Today’s so-called election in Venezuela is an insult to democracy. It’s time for Maduro to go,” the US said.

The president of Chile, Sebastián Piñera, tweeted that his country, “like the majority of democratic countries”, would not recognise the vote. “Venezuela’s elections do not meet the minimum standards for a true democracy. They are not clean or legitimate elections and they do not represent the free and sovereign will of the Venezuelan people,” Piñera wrote.

Claudio Fermín, the campaign chief for Maduro’s main opponent, Henri Falcón, claimed his team had documented 900 cases of voting irregularities involving benefits being offered outside polling stations to those who backed the president. “It’s time to remove this cheating government,” Falcón tweeted.

Addressing reporters at the presidential palace, Miraflores, earlier in the day Maduro played down those allegations as “small events” that would not affect the overall result. He claimed international criticism was part of a “permanent” campaign of aggression by “the United States’ Klu Klux Klan government”.

Maduro is hoping to secure a second six-year term despite leading his oil-rich nation into a shattering economic depression that has prompted one of the worst migration crises in recent Latin American history.

Venezuela, South America’s fifth most populous nation, has hyperinflation, crime and chronic shortages of food and medicine. Critics accuse Maduro of turning a once vibrant democracy into an increasingly dictatorial state.

But as he cast his vote in the capital, Caracas, on Sunday morning Maduro claimed a veritable democratic “fiesta” was under way.

“Today is a historic day that no one will forget,” the 55-year-old former bus driver and foreign minister told Telesur, Venezuela’s state-run broadcaster. “It’s a day of celebration. We are celebrating the freedom that we enjoy in this country … of opinion, of action, of mobilization, of thought – and the freedom to choose.”

Maduro loyalists parroted that message as they headed out to vote for the man who inherited Hugo Chávez’s ‘Bolivarian revolution’ after his death in 2013. “Look at the people in the streets!” Delcy Rodríguez, the president of Maduro’s widely reviled constituent assembly, declared. “A big democratic fiesta is taking place in Venezuela.”

Critics ridiculed those claims, denouncing the vote, which Venezuela’s mainstream opposition is boycotting and the United States has called a sham, as a “mojiganga” (farce). “These are not elections. They are a farce intended to keep Maduro in power without popular support,” Juan Pablo Guanipa, a prominent opposition leader, told Reuters.

The US secretary of state, Mike Pompeo, tweeted: “Sham elections change nothing. We need Venezuelan people running this country … a nation with so much to offer the world.”

Amid reports that polling centres had failed to open in some opposition areas and of eerily quiet streets, Falcón, issued a final plea for voters to exercise their democratic right. “The day has come to make history and to save Venezuela,” he tweeted.

That looks unlikely, with experts predicting the lowest voter turnout since Chávez was first elected in 1998 will help return Maduro to power despite the social, political and economic turmoil gripping his country.

In a televised address on the eve of Sunday’s vote, Maduro rejected claims Venezuela was “in ruins” as an imperialist smear campaign designed to topple his government.

“[Instead of attacking Venezuela] why don’t they deal with the humanitarian crisis in Africa: the unemployment, the hunger, the poverty, the lack of education and services?” he asked. “Why don’t they deal with the killing and the crisis in Gaza and the murders of dozens of men and women who are fighting for their land in Palestine?”

The Republican senator Marco Rubio joined a growing chorus of criticism on Sunday evening, tweeting in Spanish: “The dictator Nicolas Maduro isn’t fooling anyone. The world has seen this farce and supports the people of Venezuela in their struggle for the return of a democratic country.”