With 94% of votes counted, 54% backed an end to term limits, a National Electoral Council official said.
Mr Chavez has said he needs to stay in office beyond the end of his current term in 2012 so he can secure what he calls Venezuela's socialist revolution.
Critics say that would concentrate too much power in the presidency.
"The doors of the future are wide open," Mr Chavez shouted from the balcony of the Miraflores presidential palace after the results were announced.
"In 2012 there will be presidential elections, and unless God decides otherwise, unless the people decide otherwise, this soldier is already a candidate."
Crowds of the president's supporters filled in the streets, letting off fireworks, waving red flags and honking car horns.
'Revolution saved'
The BBC's Will Grant in the Venezuelan capital, Caracas, says this was the kind of strong confirmation of his socialist agenda at the polls that Mr Chavez had been seeking. This has been the most unequal, the most abusive campaign of all
Leopoldo Lopez
Opposition leader
Tough times ahead for Chavez
"This victory saved the revolution," said Gonzalo Mosqueda, a 60-year-old shopkeeper, sipping rum from a plastic cup outside the palace.
"Without it everything would be at risk - all the social programs, and everything he [Chavez] has done for the poor," he told AP.
More than 11 million voters out of almost 17 million who were eligible took part in Sunday's referendum, said the head of the electoral body, Tibisay Lucena.
Election observers, who included representatives from Latin American nations, European Parliament deputies and European academics, said the ballot had been free and fair.
Opposition figures said they would not contest the result but they said victory had been achieved thanks to huge government funding and blanket state television coverage.
Opposition leader Leopoldo Lopez told the BBC's Newshour programme on Sunday that the campaign had been heavily weighted towards Mr Chavez.
"In 10 years we have had 15 elections, 15, and this has been the most unequal, the most abusive campaign of all.
"So that's why you are seeing more propaganda, more campaigning, more advertisement for the 'yes' vote."
