Gunmen in Venezuela shot into a crowd of voters on Sunday, activists said, killing one woman and wounding three others during an unofficial referendum organised by the opposition to push for an end to two decades of socialist rule.

The opposition Democratic Unity coalition said a pro-government "paramilitary" gang opened fire in Caracas' poor neighbourhood of Catia, where thousands were participating in the opposition event. Video footage showed people scattering as gunshots rang out, many taking sanctuary inside a church.

"The day was stained by the killing of a Venezuelan woman who protesting and exercising her rights," said opposition leader Freddy Guevara of the killing of Xiomara Escot. "But violence cannot hide what has happened. The people are not afraid and are clear in their decision."

Sunday's symbolic poll, which asked voters' opinion on President Nicolas Maduro's plan for a controversial new congress, was aimed at denting his legitimacy further amid a crippling economic crisis and months of anti-government protests in which some 100 people have been killed.

Maduro, 54, has denounced the plebiscite as illegal and meaningless. Instead, the former bus driver and union leader is campaigning for an official July 30 vote in support of the proposed new assembly, which would have the power to rewrite the constitution and dissolve state institutions.

The opposition cast Sunday's unofficial referendum as an act of civil disobedience that will be followed by "zero hour," a possible reference to a national strike or other escalated actions against the president.

Lines formed early at makeshift polling stations at theatres, sports fields, and traffic circles in the oil-rich nation of 30 million as Venezuelans furious over food shortages and rampant inflation sought to make their voices heard.

There was a festive atmosphere under the Caribbean sun in most places, with people blasting music, honking car horns, waving Venezuelan flags, and chanting "Yes we can!"

More than five million people had cast ballots at 2,000 centres, the opposition said, as voting was extended into early evening so everyone still waiting in line could have their say.

"Maduro has done everything very badly, and now, via a fraudulent constituent assembly, he wants to gain time, but his time is up," said shopkeeper Rafael Betancourt, voting in late leftist leader Hugo Chavez' home state of Barinas, which has flipped to the opposition.

"This is the proof that the people will kick out whoever submits us to hunger and despair," he added, as hundreds waited to cast their ballot.

Crowds gathered to vote in other former "Chavista" strongholds too, such as the poor slums of Caracas.

Despite a strong turnout, the opposition vote does not appear to augur a short-term change of government or a solution to the country's political stalemate.

Voters were asked if they reject the proposed new assembly, whether they want the armed forces to defend the existing constitution, and if they want elections before Maduro's term in office ends in 2018.

DIASPORA VOTES

The vote also included participation by swelling ranks of Venezuelans who have moved abroad, from Miami to Madrid, to escape the OPEC nation's dire economy.

Some public employees in Venezuela, under government pressure not to participate in opposition events, sought creative ways to vote without being noticed, such as travelling across town or even going in disguise.

The opposition promised results by Sunday evening.

But they could not use the country's electoral infrastructure for the hastily convened vote. The elections council -- which the opposition calls a pawn of Maduro -- was simultaneously holding a test-run for the July 30 vote.

Also, state telecommunications regulator Conatel ordered radio and TV stations not to use the word "plebiscite" on air and told them to pull opposition ads for the vote, according to Venezuela's main organization of media workers.

There were also lines for the July 30th test run in Caracas on Sunday, though the mood was notably more subdued than at the opposition polling stations.

"We poor are going to be represented now," said retirement home worker Iraiz Alfonzo, 45, as she stood in line for the assembly vote test run.

A high opposition turnout would reflect widespread national dissatisfaction with Maduro and bolster the campaign to remove him, while low attendance would give the ruling Socialist Party a boost for the constitutional assembly.

In a phone call to state TV, Maduro acknowledged the opposition event but called it an "internal consultation."

"I urge the opposition: 'Don't go crazy, calm down.' As president of the republic, I make a call for peace," he said.

Sunday's referendum came against the backdrop of near daily anti-government protests. Masked youths with stones, Molotov cocktails and homemade mortars have battled riot forces using tear gas, water cannons and rubber bullets since April.

The unrest has resulted in fatalities among both demonstrators, government supporters and security forces, mostly from gunshots, as well as hundreds of arrests and thousands of injuries.

Maduro has refused to recognise the authority of the National Assembly since the opposition won control of it in a 2015 landslide election, which his critics call evidence he is eroding democratic institutions in order to retain power.

He says the country is the victim of an "economic war" and that opposition protests are an effort to overthrow him with U.S. connivance, which the United States has denied.