Opposition candidate Henrique Capriles, who is likely to win Sunday's poll, cast his vote early in the capital [Reuters]

Venezuelans have lined up to vote in the country's first-ever opposition presidential primary, choosing a single challenger who will run against Hugo Chavez, the country's president for 13 years.

With all of Venezuela's 18 million registered voters able to take part, all eyes were on turnout as the clearest test yet of the opposition's strength and chances of ending Chavez's rule.

"The turnout has been huge," said Al Jazeera's Lucia Newman, reporting from an opposition stronghold area of Caracas.

"The polls were supposed to close about an hour ago, but it is unlikely that they will be able to because the turnout is so high."

The poll outcome will set the stage for what many are billing as the most anticipated presidential contest since Chavez's first electoral triumph in 1998, and Venezuelans on both sides of the nation's political divide are eager to see who will emerge as the challenger.

Henrique Capriles, the Miranda state governor, leads preliminary polls with as many as 20 per centage points over another governor, Pablo Perez, 42, and three other candidates.

"I aim to be a president who talks much less, who doesn't invade Venezuelans' personal lives so much," Capriles said in a pointed reference to Chavez's longwinded speeches, which local media are often obliged to run live.

"The real game begins on February 13," he said.

Capriles hails Brazil's market-friendly but socially conscious policy model as his inspiration and has said he would take a "no shocks" approach to dismantling Chavez's statist economic policies such as currency controls.