The opposition has urged defiance of the protest ban by calling for demonstrations to intensify.
The run-up to the vote has been marked by months of clashes between protesters and the government that have left at least 116 dead and almost 2000 wounded.
One voter on Sunday, Ana Contreras, said: "I have come to vote to tell the gringos and the opposition that we want peace, not war, and that we support Maduro". Maduro dispatched National Guard soldiers and "colectivos", armed groups of pro-government vigilantes.
Venezuelans are coming Sunday to the polls for a key election to choose representatives to the National Constituent Assembly (NCA).
President Nicolas Maduro ordered the vote - aimed at selecting a so-called "constituent assembly".
The opposition has called for a boycott and mass demonstrations against the election, which it calls a bid by Maduro to install a dictatorship with the backing of the military.
The United States, who already impose sanctions against Venezuela, stepped up the pressure with President Trump promisingexternal link "strong and swift economic actions" if the assembly comes into effect.
That power has led to fears Maduro will again try to dissolve the current National Assembly and arrest his critics, something he's promised to do.
In this Monday, July 24, 2017 photo, a demonstrator dressed as Venezuelan independence hero Simon Bolivar is silhouetted against a national flag during a tribute to those killed in the recent wave of anti-government protests, in Caracas, Venezuela. Across the city, residents said they wanted President Nicolas Maduro out of power but didn't want to risk their lives or livelihoods taking on his government.
"No!" the crowd shouted back.
The special assembly being selected Sunday will have powers to rewrite the country's 1999 constitution but will also have powers above and beyond other state institutions, including the opposition-controlled congress. That constitution is considered one of his principal legacies, and the move to rewrite it has drawn rebuke even from some longtime government loyalists and Chavez supporters.
CGTN's Juan Carlos Lamas has more from Caracas.
The government held simulated elections two weeks ago, to show citizens how to mark their ballots. The decision was quickly reversed but it sparked a protest movement demanding a new presidential election. Clashes with police began Friday afternoon and lasted into the night.
In a statement to AFP blasted by Maduro for becoming public, Spain's former prime minister Jose Rodriguez Zapatero said that dialogue in Venezuela "could bring an end to the grave crisis".
Air service to Venezuela continued to dwindle.
"They took his violin and hit him with it", Romero said. Avianca was offering full refunds to the estimated 13,000 passengers who had booked a flight on the now-suspended service. The airline declined further comment.
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