Fireworks lit up the night sky in Caracas and supporters near the presidential palace roared their approval when electoral authorities announced that Chavez won a referendum by 58% to 42%.
But the Venezuelan opposition has dismissed the numbers, claiming "gross manipulation" of the ballot results.
Chavez, a baseball fan, said he had scored a home run against the US with his victory in a referendum on his mandate, joking from the balcony of the presidential palace in Caracas: "They informed me the ball fell in the centre of the White House."
The Venezuelan leader, who often rails against "imperialist" Washington, has accused US President George Bush of financing the opposition's efforts to drive him from office.
The opposition has rejected the official results, however, claiming there was massive fraud during the voting.
"We categorically reject the results," said Henry Ramos Allup, spokesman for the Democratic Coordinator opposition coalition.
"We categorically reject the results"
Henry Ramos Allup,
Allup said the incumbent got only 40% of the vote, with 59 calling for his mandate to be revoked.
He said the opposition remained determined to seek a peaceful solution and that "not even fraud or gross manipulations like this one will alter our convictions".
A self-proclaimed champion of the poor, Chavez says his opponents are "oligarchs" seeking the restoration of privileges they used to enjoy in the oil-rich but poverty-stricken South American country.
Venezuelans on Sunday and Monday voted in droves.
Aljazeera's correspondent says
voter turnout has been massive
"There has been a massive voter turnout," said Aljazeera's correspondent in Caracas, Dima al-Khatib.
Officials twice postponed the polling stations' closing time, but Chavez himself had said voting centres would stay open as long as citizens were still waiting to vote.
One person died and 10 others were wounded after unknown armed assailants fired on people queuing up to vote just outside the capital on Sunday.
Held amid a politically charged atmosphere, the referendum has heightened passions and split Venezuela between pro- and anti-Chavez forces.