Gunmen on motorbikes attacked Venezuelan voters lining up for an opposition-organized ballot challenging President Nicolas Maduro on Sunday, killing a 61-year-old woman and wounding three other people, according to prosecutors.

Television footage of the 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 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. The oil-rich nation is suffering its worst ever economic crisis, with an acute shortage in basic goods and the world's highest inflation rate. More than 90 people have been killed in anti-government protests that have taken place almost daily since April.

Maduro, who has refused to step down, also has called his own vote later this month to elect the special assembly. The opposition sees his plans for constitutional change as an attempt to cement his grip on power.

Hundreds of thousands of Venezuelans took part in Sunday's vote, which asked whether they supported Maduro's plans to have a citizens' body elected on July 30 to carry out the constitutional do-over. With government supporters boycotting the process, the result looked likely to be an overwhelming rejection.

Maduro warned the opposition "don't go crazy" over its vote, after the head of the national electoral council said the symbolic poll was of "no legal consequence."

The president urged the opposition to "come over to peace, to the constitution," telling officials who were setting up his July 30 election that differences "must be resolved in peace, with ballots, not bullets."

Several Latin American countries and the Catholic Church have criticized Maduro's move to a new constitution, which the opposition says is a bid by Maduro to concentrate dictatorial prerogatives.

With the opposition and the government dug in on their respective courses, dialogue -- attempts at which had already failed -- appeared a distant prospect.

The population in the oil-rich South American nation, meanwhile, 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 legislature. The president, in turn, says the opposition is collaborating with the "imperialist" United States to undermine to topple his government. He says his proposed 545-member Constituent Assembly is "the only path" to peace and economic recovery.