The poll today is meant to gauge public support for a plan by Maduro to rewrite the constitution through the election on July 30 of a citizens' body.
But with authorities refusing to greenlight Sunday's vote and pro-Maduro supporters boycotting it, it looked likely to return a rejection of the president's scheme.
Likewise, the opposition has told its supporters to stay away from the July 30 election.
The cross-purpose initiatives have given rise to international worries -- voiced by the Catholic Church and the head of the UN, Antonio Guterres -- that the chances of bringing both sides together for dialogue has become more remote.
While Maduro is deeply unpopular -- with 80 percent of Venezuelans criticizing his rein, according to the Datanalisis survey firm -- he enjoys backing from some, mostly poor, parts of the population and, most importantly, from the military.
Many Venezuelans, though, are less focused on the political powerplay than they are on getting by day-by-day under their country's crushing economic crisis, which has meant shortages of food and medicine.
"Everything is ready," one opposition figure, Maria Corina Machado, told AFP.
She predicted today's vote would "not only reject the Constituent Assembly" -- the body Maduro is seeking to have elected to come up with a new constitution -- "but will give a mandate for a change of the regime, the end of the dictatorship and the start of a transition with a government of national unity."
But Maduro, giving a national radio and TV broadcast, portrayed the vote as merely an "internal consultation by the opposition parties" with no electoral legitimacy.
He directed his followers instead towards a rival poll exercise that, unlike that of the opposition, has been approved by electoral authorities: a dry-run simulation of the election to take place on July 30.
He also repeated claims the opposition was tied to foreign powers -- implied to be the "imperialist" United States -- with the aim of toppling his government.
The international media, he railed, was covering the opposition vote in a way to justify foreign intervention.
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