The opposition has called for close monitoring of an outcome they predict will be close.
Some 140,000 soldiers and reservists were being posted during the vote, the Defense Ministry said.
About 100 electoral observers from 39 countries in Latin America, Europe and the US will be on hand, plus hundreds of Venezuelan observers, according to the National Electoral Council.
Chavez, 53, says he will stay in power only as long as Venezuelans keep re-electing him -- and adds that might be for life.
"If God gives me life and help," Chavez told supporters on Friday, "I will be at the head of the government until 2050!" -- when he would be 95 years old.
Meanwhile, former US defense secretary Donald Rumsfeld warned yesterday that the constitutional referendum in Venezuela threatened to completely destroy the country's democracy and called Chavez an "aspiring despot."
"Today the people of Venezuela face a constitutional referendum, which, if passed, could obliterate the few remaining vestiges of Venezuelan democracy," Rumsfeld wrote in an essay published in the Washington Post.
Rumsfeld said the Venezuelan leader in fact is trying to dismantle Venezuela's Constitution, silence its independent media and confiscate private property.
Also, Venezuela will nationalize Spanish banks and expel the Spanish oil company Repsol if conservatives win power in Madrid in general elections in four months' time, Chavez said on Saturday.
He made the warning in a news conference in which he also softened slightly his stance against Spanish King Juan Carlos, who nettled him at a recent summit by telling him to "shut up."
"If Spain's conservatives, who continue to attack us, end up governing, then give me your banks, give me the bill," he said.
Asked about a threat he made at late on Friday about nationalizing Spanish banks if Juan Carlos did not apologize for his remark, Chavez backtracked.
"We don't want to worsen the situation with the Spanish government. It's possible there will be contact with it in the coming days," said the president, who has put bilateral relations "in the freezer."
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