Activists say the armed men were seeking to disrupt an unofficial referendum organised by the opposition against government plans to change the nation's constitution.
Lydia Feng reports.
Thousands of Venezuelans have lined up across the country and abroad to vote in a symbolic rejection of President Nicolas Maduro's plan to rewrite the constitution.
Opposition politicians organised the unofficial poll, held in improvised polling stations at theatres, sportsgrounds and roundabouts in Venezuela and over 100 other countries.
President Maduro has described the vote as meaningless.
But opposition leader Henrique Capriles called for as many people as possible to vote in a move to dent the government's legitimacy.
"What should happen in the coming hours -- after what has been such an overwhelmingly peaceful, democratic and constitutional expression by the Venezuelan people -- is the cancellation of the fraudulent constituent. Constitutional order needs to be re-established in our country."
An official referendum is set for July the 30th over whether to accept a new assembly, which would have the power to rewrite the constitution and dissolve state institutions.
But the opposition says it believes the new assembly could herald dictatorship.
It says the government's plan is structured to fill the assembly with government supporters and allow President Maduro to eliminate the few remaining checks on his power.
There was an air of optimism during the referendum, but it was marred by armed men reportedly swarming a polling site in Caracas and firing shots at the crowd.
A 61-year-old woman was killed.
Video posted to social media showed massive crowds outside a church, then hundreds of people running in panic as motorcycle-riding men zoomed past and shots rang out.
The shooting is a reminder of just how heated politics is in the country.
Another opposition leader, Leopoldo Lopez, under house arrest after recently being released from prison, posted a video on social media of him filling out his ballot at home.
He was jailed on charges of inciting violence during deadly protests in 2014 and remained jailed until his surprise release this month.
His wife, Lilian Tintori, says she is optimistic about the outcome of the vote.
"My response to today's referendum is, 'Yes, yes, yes! Yes to freedom. Yes to democracy. Yes to respecting human rights. Yes to the people deciding.' Leopoldo did the same thing a few minutes ago. He's already voted."
Several former Latin American presidents visited Venezuela to observe the referendum being carried out.
Former Colombia president Andres Pastrana called on Venezuelans to use their vote to undermine President Maduro's plans.
"No to the constitutional assembly, and, as such, the constitutional assembly must be abandoned. And, secondly, I reiterate that the most important thing is that, if the National Assembly itself called a referendum and that referendum gives a mandate, that assembly is the one that, tomorrow, must start implementing that mandate that the people have given it."
