Mr Capriles, who lost by a very narrow margin, said there were more than 300,000 incidents from Sunday's poll that needed to be examined.

Socialist Nicolas Maduro has called for the results to be respected.

The election was called after Mr Chavez's death from cancer on 5 March.

Mr Maduro, a former bus driver whom Mr Chavez had named as his preferred heir, won 50.7% of the vote against 49.1% for Mr Capriles.

The National Electoral Council said the results, which it announced on Sunday night, were "irreversible".

Russian President Vladimir Putin and Cuban leader Raul Castro were among the first heads of state to congratulate Mr Maduro on his win.

As the news of Mr Maduro's victory emerged, celebrations erupted in the capital, Caracas.

Thousands of jubilant supporters took to the streets, dancing, singing and blasting car horns, while fireworks lit up the night sky. Opposition voters banged pots and pans in protest.

Speaking outside the presidential palace, Mr Maduro told crowds that the result was "just, legal and constitutional".

He said his election showed Hugo Chavez "continues to be invincible, that he continues to win battles''.

Mr Maduro, who was wearing a tracksuit top in the colours of the Venezuelan flag, said he had spoken to Mr Capriles on the phone, and that he would allow an audit of the election result.

He called for those who had not voted for him to "work together" for the country.

But Mr Maduro's margin of victory was far narrower than that achieved by Chavez at elections last October, when he beat Mr Capriles by more than 10 percentage points.

Almost immediately one member of the National Electoral Council who does not have government sympathies called on the authorities to carry out a recount by hand, a call later echoed by Mr Capriles himself.

At Mr Capriles' campaign headquarters the mood was sombre, as his supporters watched the results on television. Some cried, while others hung their heads in dismay,

Shortly afterwards, Mr Capriles emerged, angry and defiant.

"It is the government that has been defeated," he said. Then, addressing Mr Maduro directly, he said: "The biggest loser today is you. The people don't love you.

"Mr Maduro, if you were illegitimate before, now you are more so."

The new president faces an extremely complex task in office, says the BBC's Central America correspondent, Will Grant.

Venezuela has one of the highest rates of inflation in the region and crime rates have soared in recent years, particularly in Caracas. Food shortages and electricity blackouts are also common.

But perhaps Mr Maduro's biggest challenge will be trying to govern a country which is so deeply divided and polarised, and where the opposition say they have an increasingly legitimate stake in the decision-making process, our correspondent says.

The closeness of the race has also caused introspection inside Mr Maduro's own United Socialist Party (PSUV).

The man considered to be Mr Maduro's main rival, National Assembly President Diosdado Cabello, tweeted that the results "oblige us to make a profound self-criticism".

Divisive legacy

Mr Maduro had been serving as acting president since Mr Chavez died.

He is due to be sworn in on 19 April and serve until January 2019 to complete the six-year term that Mr Chavez would 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 accused him of being an autocrat, intent on building a one-party state.

At the scene

Irene Caselli