People in Venezuela have been voting in a presidential election, called after the death of Hugo Chavez.
Acting President Nicolas Maduro, chosen by Mr Chavez as his successor, is running against Henrique Capriles, currently governor of Miranda state.
Mr Capriles lost to Mr Chavez in elections last October.
In a Twitter message sent as polls began to close, Mr Capriles suggested there was an attempt under way to doctor the results.
"We warn the country and the world of the intention to change the expressed will of the people!," he wrote.
Electoral authorities say voting has been going smoothly. Security had been stepped up for the vote, the BBC's Will Grant in Caracas reports.
The government said there were reports of over a dozen minor irregularities, some of which were flagged up by opposition officials.
On the eve of polls opening, Mr Capriles had accused Mr Maduro of breaking election laws by continuing its campaign on state television.
Mr Maduro, aged 50, whose campaign has focused on his close relationship to Mr Chavez, was shown visiting the tomb of the late leader, a move Mr Capriles, 40, said was "violating all the electoral norms".
Both candidates have to some extent broken the media silence they are supposed to have maintained since campaigning officially ended on Thursday, our correspondent says.
Almost 19 million Venezuelans have the right to vote in the poll.
Mr Maduro cast his vote in the Catia area of the capital Caracas, accompanied by Mr Chavez's two daughters. Mr Capriles voted in the Las Mercedes district of the capital.
Hundreds of election monitors are present from different countries and international organisations to ensure the poll is free and fair.
Voting is electronic - one machine will identify voters' fingerprints, and a second will recognise identity card numbers and register the vote anonymously.
Polls have officially closed but will stay open until all those queuing at closing time have voted.
Official results are expected about three hours after counting begins.
Our correspondent says it remains to be seen whether there is voter apathy or a real desire now to make a decision - either this Bolivarian revolution is continued or the country turns a different corner.
Late on Sunday, Mr Maduro's Twitter feed and campaign website were hacked by a group calling itself Lulz Security Peru.
The government said that it temporarily shut down the internet nationwide to stop the hacking of other officials' Twitter accounts.
The former president died on 5 March, after a two-year battle against an undisclosed type of cancer, prompting a short electoral campaign period before Sunday's elections.
The winner is due to be sworn in on 19 April and serve until January 2019, to complete the six-year term that Mr Chavez was supposed to have begun in January.
Mr Chavez was a divisive leader. To his supporters he was the reforming president whose idiosyncratic brand of socialism defeated the political elite and gave hope to the poorest Venezuelans.
He effectively used his country's vast oil reserves to boost Venezuela's international clout, and his strident criticism of the US won him many political allies in Latin America.
However, his political opponents accuse him of being an autocrat, intent on building a one-party state.
Mr Chavez bequeaths a nation beset by crumbling infrastructure, unsustainable public spending and under-performing industry.
His handpicked candidate Nicolas Maduro is seen as the front-runner, but recent polls suggested the gap between him and his rival was narrowing.