Supporters of Venezuelan President Nicolas Maduro shout slogans waiting for the announcement of the election results in Venezuela, on May 20, 2018.

Venezuela's leftist leader Nicolas Maduro won a new six-year term on Sunday, but his main rivals disavowed the election alleging massive irregularities in a process critics decried as a farce propping up a dictatorship.

Victory for the 55-year-old former bus driver, who replaced Hugo Chavez after his death from cancer in 2013, may trigger a new round of western sanctions against the socialist government as it grapples with a ruinous economic crisis.

U.S. President Donald Trump's administration is threatening moves against Venezuela's already reeling oil sector.

Venezuela's election board, run by Maduro loyalists, said he took 5.8 million votes, versus 1.8 million for his closest challenger Henri Falcon, a former governor who broke with an opposition boycott to stand.

"They underestimated me," Maduro told cheering supporters on a stage outside his Miraflores presidential palace in downtown Caracas as fireworks sounded and confetti fell on the crowd.

Turnout at the election was just 46.1 percent, the election board said, way down from the 80 percent registered at the last presidential vote in 2013. The opposition said that figure was inflated, putting participation at nearer 30 percent.

"The process undoubtedly lacks legitimacy and as such we do not recognize it," said Falcon, a 56-year-old former state governor, looking downcast.

Maduro had welcomed Falcon's candidacy, which gave some legitimacy to a process critics at home and around the world had condemned in advance as the "coronation" of a dictator.

Falcon's quick rejection of Sunday's election, and call for a new vote, was therefore a blow to the government's strategy. Opposition abstainers saw it as vindicating their decision.

Falcon, a former member of the Socialist Party who went over to the opposition in 2010, said he was outraged at the government's placing of nearly 13,000 pro-government stands called "red spots" close to polling stations nationwide.