Vote results for Chavez and opposition candidate Henrique Capriles are expected to be close.
Capriles, a 40-year-old former state governor and lawyer, has criticized Chavez, 58, on many fronts during the campaign.
He said Chavez broke his promise to clean up the sewage-filled Guaire River in Caracas and has failed to fulfill promises to help the poor feel more secure, in light of the country's high murder rate and rising unemployment.
Capriles has united the opposition, so the votes against Chavez won't be split. He has also tapped into a sense of frustation in Venezuela.
"Has 21st-century socialism given you safety and security? Has the revolution created jobs?" he recently asked supporters.
Chavez, who has run a low-key campaign because of his battle with cancer, told a huge rally in Caracas this week that he needs a third term to consolidate social reforms and eradicate poverty.
Chavez said social programs won't survive without him as leader and he has promised to deepen socialist policies if he wins.
Capriles has painted himself a centre-leftist, but says the state-owned oil company PDVSA could be better managed. Critics of Chavez complain the company's payroll is full of patronage appointments and that government ministries are burgeoning with do-nothing jobs.
Capriles has promised to keep social programs, but says he'll stop sending free oil to Venezuela's left-wing allies and instead, will spend the money at home.
With people struggling with high inflation, violence crime and government corruption, Capriles' message is resonating in the oil-rich country.
Some polls ahead of the election found Chavez with a lead of about 10 percentage points, while other surveys showed the two candidates roughly even.
Hugo Chavez, 58, has been president since 1999. As an army paratroop commander, he led a failed 1992 coup attempt. He was jailed, later pardoned and elected president in 1998. He survived a short-lived 2002 coup. His Bolivarian Revolution movement, named after 19th-century independence hero Simon Bolivar, is moving Venezuela toward socialism, he says. Chavez has twice won re-election. His only clear electoral loss came in 2007, when voters rejected constitutional changes. Chavez announced in June 2011 that he had a cancerous tumour removed from his pelvic region. He has since undergone another surgery, chemotherapy and radiation treatment. He now says he is cancer-free.
Henrique Capriles, 40, a former state governor, won a first-ever opposition presidential primary in February. Capriles won a congressional seat at age 26. He was a Caracas district mayor and in 2008 defeated a Chavez ally, Diosdado Cabello, to become governor in Miranda state, which includes part of Caracas. Capriles describes his views as centre-left. He says he admires former Brazilian President Luiz Inacio Lula da Silva’s promotion of pro-business policies while also funding social programs for the poor.