At least one person has reportedly been killed during violence in Venezuela during an unofficial referendum held by the opposition in protest against President Nicolas Maduro's plans to rewrite the constitution.

A 61-year-old woman was killed and four people were wounded by gunfire after government supporters on motorcycles swarmed an opposition polling site in a church in the neighbourhood of Catia, just northwest of Caracas, according to reports.

Carlos Ocariz, an opposition spokesman, said: "There was an incident in Catia a short while ago in which paramilitaries opened fire."

READ MORE: Venezuela: Why are people voting today?

Local news media reported that pro-government armed groups, known as "colectivos," opened fire on the opposition supporters.

The reports said that up to 500 people had taken refuge in the church during the attack, including Catholic Cardinal Jorge Urosa.

In comments on Twitter, Henrique Capriles, Venezuela's opposition leader, accused President Nicolas Maduro and "his corrupt leadership" of sending "their paramilitaries" to attack Catia.

Vote boycotting

Sunday's plebiscite, organised by the opposition, had not been approved by electoral officials.

More than 7.1 million Venezuelans took part in the poll, the opposition said, falling short the 11 million they had hoped for.

Venezuela's total population is around 30 million.

Opposition voters look poised to reject the president's proposal to retool the 1999 constitution by electing a super-body known as the constituent assembly on July 30.

The opposition told supporters to boycott the July 30 poll, which they see as a power grab move by Maduro.

He insists the constitution needs updating to confront an economic crisis and bring peace after months of protests.

In response to the opposition's vote, the government called for its own nationwide exercise on Sunday, a rehearsal for the July 30 assembly.

Maduro portrayed Sunday's vote as merely an "internal consultation by the opposition parties" with no electoral legitimacy.

"I call on all Venezuelans to participate peacefully in political events tomorrow, with respect for others' ideas, with no incidents. Peace is what I ask," he said.

But Maduro's constitutional referendum has seen tensions rise in a nation stricken by widespread protests and unrest.

READ MORE: Venezuela - A country divided

"Analysts and universities here suspect that an unpopular government could rewrite the constitution to tighten their grip on the country, doing away with the opposition-dominated national congress and cancelling the 2018 presidential elections," said Al Jazeera's John Holman, reporting from Cucuta, in neighbouring Colombia.

"Democracy itself could be at stake," he said.

The symbolic vote was held at 2,000 polling stations across the country, and in 80 countries for Venezuelans abroad.

Foreign observers

According to Datanalisis, 70 percent of Venezuelans reject Maduro's idea of a constituent assembly.

"The purpose of the referendum is so that the public says what they actually want," Jose Sanchez, a referendum organiser, told Al Jazeera.

"If they reject Nicolas Maduro's plan, or not."

Although the referendum has no legal standing, Latin American presidents from Bolivia, Colombia, Mexico and Costa Rica flew to Venezuela at the opposition's invitation to act as observers of the vote, alongside electoral experts from various countries."This fraudulent constitutional assembly will create a majority that will shut congress, throw democracy out the window, wipe out state governors and fire the chief prosecutor," Jorge Quiroga, a former Bolivian president, told AP news agency on Saturday.

"Tomorrow, democracy and freedom are in play."

The UN has also weighed in.

"We urge authorities to respect the wishes of those who want to participate in this consultation and to guarantee people's rights to freedom of expression, association and peaceful assembly," Elizabeth Throssell, spokesperson for the UN Human Rights Office of the High Commissioner (OHCHR), said.

On Friday, Antonio Guterres, UN secretary-general, called for talks between the opposition and government.

Luis Almagro, the head of the Organization of American States, had called on Venezuelans to take part in Sunday's vote "to prevent the definitive collapse" of the country's institutions.

So far, clashes between protesters and police have left at least 93 people dead, 1,500 wounded and more than 500 behind bars.