Venezuela election: Chavez and Capriles contest poll
Incumbent President Chavez is challenged by Henrique Capriles, who represents a coalition of opposition parties.
Voters in Venezuela are going to the polls in what is predicted to be the country's most tightly contested presidential election in a decade.
Left-wing incumbent Hugo Chavez, first elected in 1998, is being challenged by opposition leader Henrique Capriles.
Mr Chavez wants to continue what he calls his socialist revolution while Mr Capriles has promised to restore economic growth.
Almost 19 million Venezuelans are eligible to vote in the election.
Mr Chavez - who is seeking a fourth term in office - was diagnosed with cancer last year but says he has now fully recovered.
A colourful and often controversial figure on the international stage, President Chavez, 58, has nationalised key sectors of the economy.
Venezuela is a major oil producer and high oil prices over the past decade have allowed his government to fund health-care, education programmes and social housing.
He says he needs another term to complete his "Bolivarian Revolution" towards socialism.
However, Mr Capriles, 40, and the opposition say the president's policies have led to bureaucracy, inefficiency and shortages.
They also accuse Mr Chavez of authoritarianism, and of suppressing the judiciary and silencing critics in the media.
Mr Capriles says a lack of investment in Venezuela's crucial oil industry has led to a decline in production.
BBC Mundo correspondents in the capital Caracas say Chavez loyalists have been using trumpets to sound a "wake-up" call for voters.
Mr Capriles' supporters were also banging pots in the street in what they called their "goodbye song" for Mr Chavez, our correspondents say.
Defence Minister Henry Rangel Silva said the armed forces had identified some groups planning to cause public disturbances but said violence was "unlikely", the state news agency AVN reported.
He also warned those who he said may be thinking of stirring up trouble that troops were on stand-by to quell any disturbances.
Queues formed early outside schools used as polling stations.
Gerardo Montemarano, who was already waiting to vote when the polls opened, brought a chair with him. "I knew there was going to be a long queue," he told the BBC.
About 100,000 Venezuelans, including about 2,000 in the UK and the Republic of Ireland, are registered to vote at diplomatic outposts around the world.
Hundreds of opposition supporters gathered outside the consulate in central London as expatriates cast their ballots.
"I don't support this government," said voter Rebecca Anaya. "I am here because I cannot live in that country. It's almost impossible. The security situation is the worst thing in the world."
BBC reporter Glen Campbell, who spent two hours outside the consulate on Sunday morning, says not a single voter identified themselves as a Chavez supporter.
However, he cautions that this is not very surprising, because by definition many of those who have left Venezuela are critics of the current regime.
Both candidates have been using social media to urge voters to cast their ballots.
Almost 140,000 soldiers have been deployed to guard more than 10,000 voting centres.
A week before the election, three opposition activists were killed during a campaign rally, while four people were injured in a shooting during a voting rehearsal in September.
From Saturday evening to Monday evening, the sale of alcohol is banned and only the security forces will be allowed to carry arms.
National Electoral Council (NEC) official Socorro Hernandez called on all parties and non-governmental groups to contribute to a peaceful election "and avoid any distortions".
While polls are scheduled to close at 18:00 local time (22:30 GMT), NEC President Tibisay Lucena said that the hours could be extended if voters were still queuing to cast their ballots.