Mr Chavez has admitted that the opposition is likely to take seats from his party Venezuelans are voting for a new parliament, with opposition parties poised to return to the National Assembly after a poll boycott in 2005.
They are set to take back seats from the governing United Socialist Party (PSUV) of President Hugo Chavez.
But Mr Chavez hopes to hold on to a two-thirds majority in the Assembly.
Opinion polls suggest that the vote could be tight in what is seen as a test of Mr Chavez’s popularity two years before presidential elections.
Mr Chavez has admitted that his party is likely to lose seats to the opposition umbrella group Table for Democratic Unity.
He appealed to Venezuela’s 17 million voters to prevent a derailment of the Venezuelan “revolution” and denounced the opposition campaign as “Operation Demolition”.
Polls opened at 1030 GMT and will remain open until 2230 GMT.
Five years ago, opposition groups boycotted the legislative elections, a decision that helped left-wing parties loyal to Mr Chavez to get almost all of the Assembly’s 165 seats.
The BBC’s Will Grant in Caracas says the opposition has since changed its rhetoric.
Rather than concentrating on their dislike for the president, the opposition has kept its campaign narrowly focused on issues like crime and the rising cost of living, our correspondent says.
Official statistics show that around 14,000 murders were committed in Venezuela last year, more than twice the number of murders than in 1999, the year Mr Chavez came to power.
The government disputes the figure, and Mr Chavez recently said on television that it was “simply not true that Venezuela is one of the most dangerous countries in the world”.
Criticism of recent water and power shortages have also been part of the opposition campaign.
There have been heavy rains in Venezuela over the last few days with several people killed in landslides.
If the downpours continue on Sunday, that could affect turnout, our correspondent says.
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