Several people have been killed and protesters have clashed with security forces as Venezuelans vote for a new assembly with powers to rewrite the constitution.
A candidate in the assembly elections and an opposition activist are among those killed.
Opposition parties are boycotting what they say is a rigged election and a power grab by President Nicolás Maduro.
Mr Maduro, widely disliked for overseeing an economic collapse during four years in office, has pressed ahead with the vote to create the all-powerful assembly despite the threat of further US sanctions and months of opposition protests in which more than 115 people have been killed.
The government says the constituent assembly is the only way to bring peace back to the country.
José Felix Pineda, a 39-year-old lawyer running in the election, was shot in his home on Saturday night local time, a senior Venezuelan minister said.
Ricardo Campos, a youth secretary with the opposition Acción Democrática party, was shot dead during a protest, the head of the national assembly said.
Venezuelan newspaper El Universal said at least three other men were killed in protests in Merida, in the west of the country.
Authorities confirmed three deaths in total, Reuters reported, including the killing of a candidate during a robbery, although opposition lawmakers said the toll was much higher.
Protesters have blocked roads in the capital in defiance of a ban on demonstrations.
The security forces have used armoured vehicles to dispel protesters in the Caracas district of El Paraíso amid the sound of gunfire, AFP says, citing local reports.
Mr Maduro, a former bus driver and union leader narrowly elected in 2013, has accused right-wing governments of trying to sabotage 21st century socialism.
"The 'emperor' Donald Trump wanted to halt the Venezuelan people's right to vote," said Mr Maduro, as he rapidly voted at 6am in a low-income area of the capital Caracas that has turned on the government.
"A new era of combat will begin. We're going all out with this constituent assembly," he added.
In a speech broadcast on TV, President Maduro predicted a "big victory", calling the vote "the most important election held in Venezuela's political system".
However, electoral council chief Tibisay Lucena acknowledged that some voting machines had been attacked and burned in parts of the country.
The election has been heavily criticised by other Latin American countries as well as the European Union and the United States, which last week imposed new sanctions on members of Mr Maduro's government.
Venezuela, a major oil producer, is gripped by a political crisis, with soaring inflation and daily food shortages.
Many residents in Caracas were stocking up on essential items on Saturday in case unrest left shops closed into Monday, correspondents said.
The vote: key points
- President Maduro wants to form a new constituent assembly, that will have the specific purpose of drafting or adopting a new constitution
- He said the new assembly would promote "reconciliation and peace", but did not give details of exactly how rewriting the constitution would achieve such broad aims
- He also said that the constituent assembly would promote dialogue in the polarised country
- The opposition says it is a way for President Maduro to maximise his power and cling on to it for longer, as well as delay future elections
- Some loyalists of late President Hugo Chávez are also unhappy, as they treasure the old constitution drafted in 1999
- BBC / Reuters