Gunmen on motorbikes attacked Venezuelan voters lining up for an opposition-organized ballot challenging President Nicolas Maduro, killing a 60-year-old woman and wounding three other people, according to prosecutors.
Television footage of Sunday's attack showed a panicked crowd running and screaming while gunshots were heard. Many sought refuge in a nearby church.
The armed assault occurred in Catia, a working class neighborhood in the west of the capital Caracas. Prosecutors said an investigation had been opened.
The opposition coalition blamed "paramilitary groups" linked to Maduro's government.
The violence confirmed fears surrounding an electoral tussle between the opposition and Maduro, focused on the beleaguered president's bid to rewrite the constitution.
It also fed into nearly four months of protests during which almost 100 people have died.
Hundreds of thousands of Venezuelans cast ballots on Sunday in the vote aimed at rejecting Maduro's plans to have a citizens' body elected on July 30 to carry out the constitutional do-over.
The call to polls was described as a "plebiscite" by the opposition, but slammed as "illegal" by the government.
The opposition predicted a big turnout, and lines remained in front of polling tables as they were closing.
To detract from the opposition vote, Maduro's government held a dry-run simulation on the same day of the election to be staged at the end of the month.
Several Latin American countries and the Catholic Church have criticized the move to draw up a new constitution, which the opposition says is a bid by Maduro to concentrate dictatorial powers to stay in power.
Voters wearing white or the colors of the Venezuelan flag lined up in front of the 14,000 makeshift polling tables set up under tents across the country by the opposition.
"We are turning out to show our discontent with the government," 49-year-old Tibisay Mendez said in Caracas.
"We can't find medicines, every day there is less food in the country. And they (Maduro and his officials) only want to hold on to power. We are voting to get them out," she said.
Voters were asked if they rejected Maduro's plan to create a "Constituent Assembly" to redo the constitution, and if they approved of early elections.
Julio Borges, leader of the opposition-controlled parliament, said the vote was a watershed moment "in this fight to win back democracy for Venezuela."
But the head of the National Electoral Council, Tibisay Lucena, cautioned the opposition against creating "false expectations" with its vote, saying it was of "no legal consequence."
Maduro told the opposition "don't go crazy" over the vote and urged them to "come over to peace, to the constitution."
Differences "must be resolved in peace, with ballots, not bullets," he said in a call to officials preparing the Constituent Assembly poll.
Both the opposition and the government have dug in on their respective courses and previous attempts at dialogue have failed.
Meanwhile the population is suffering a crushing economic crisis, with shortages of food and medicine and triple-digit inflation.
The opposition accuses Maduro of driving the country into bankruptcy, and of using the Constituent Assembly to entirely sideline the parliament.
The president, in turn, says the opposition is collaborating with the "imperialist" United States to undermine the economy, and to topple his government.
He has presented his proposed 545-member Constituent Assembly, drawn from various sectors of society, as "the only path" to peace and economic recovery.
Although he is deeply unpopular -- the Datanalisis polling firm says 80 percent of Venezuelans reject Maduro as leader -- he retains the loyalty of military chiefs, and can count on the support of electoral authorities and judges.
But some cracks are appearing in his camp. The most stark was the rebellion of Attorney General Luisa Ortega, who came out against the Constituent Assembly. She holds onto her office, for now, remaining a potent voice of dissent in government.
According to Datanalisis, 70 percent of voters taking part in Sunday's consultation were expected to reject the Constituent Assembly. It expects participation to reach 10.5 million people -- a majority of Venezuela's 19 million voters.