Venezuelan opposition parties are holding their first primary on Sunday to pick a unity candidate to battle ailing President Hugo Chavez, in power for more than a decade, in an October vote.

Five candidates are running in the opposition contest with the favorite Henrique Capriles, 39, the energetic governor of Miranda state, polls show.

The 57-year-old Chavez, who last year underwent chemotherapy in Caracas and Havana and now claims to be cancer-free, is seeking a third six-year term in the October 7 vote.

A fiery critic of the United States, Chavez is the main political and economic ally of Cuba, the only one-party communist regime in the Americas.

Capriles, telegenic and energetic, has been in politics since 25. His campaign got a boost last month when Leopoldo Lopez, a popular former mayor, dropped out and endorsed him.

He describes his politics as center-left, and has argued that Venezuela can "replicate" Brazil's model of economic development: allowing markets to play their role, while also making social progress a priority.

Capriles is also known for having confronted Chavez back in 1999, when the governor was a lawmaker.

His main opposition rival is Pablo Perez, 42, of the Un Nuevo Tiempo (A New Era) party. Perez governs Zulia, Venezuela's most populous and wealthiest state.

Both Perez and Capriles say they want to end the country's deep political polarization and have pledged to fight poverty. They have campaigned with a conciliatory message and have avoided directly criticizing Chavez.

They also propose continuing and improving the popular social programs adopted by the Chavez government since 2003, notably in health and housing.

The other candidates in the race are independent legislator Maria Corina Machado, labor leader Pablo Medina, and former ambassador Diego Arria. Unlike the governors, these three have chosen to aggressively challenge Chavez.

"This (vote) is not about changing the president. It is about changing a model that has failed," Capriles said on the campaign trail this week, putting the spotlight on changing the economic and social landscape even though his pledge has not been to eliminate socialist Chavez' most popular programs.

A lawyer with movie-star looks whose family includes Jewish immigrants, and whose musical tastes run to danceable reggaeton, Capriles would seem to embody a generational change as well as a political one.

Capriles and Perez have emerged in recent months "as favorites precisely because they sought to depolarize the country and refrained from confronting Chavez," said historian Margarita Lopez Maya.

"It's apparently an electoral strategy that works," she added.

Perez said he does not plan to roll back all of Chavez policies.

"We don't intend to come to power and say: We are ending everything and bringing something else," he said. "What we view as good, we'll keep, what needs to be improved we will improve and with what we disagree, we will see."

The US-backed coalition has called on Venezuelans to head to 7,600 polling stations set up around the country to cast ballots, and have vouched for the confidentiality of their votes.

In January the opposition parties unveiled a unity platform focusing on free-market economics and emphasizing public safety.

This would include an end to price controls, in place since 2003; adoption of a competitive currency exchange rate; reassessing Chavez' creation of a socialist state; and returning autonomy to the Central Bank.

A key issue will be voter turnout.

The primary is the first of its kind and it remains to be seen what turnout can be rallied. Balloting is also for potential opposition governors and mayors.

Observers will be on hand from countries including Spain, Colombia, Peru, the United States, Australia and Japan.