CARACAS (AFP) - Venezuelan opposition parties voted Sunday in their first primary to pick a unity candidate to battle ailing President Hugo Chavez, in power for more than a decade, in an October vote. Balloting began throughout the country at 1230 GMT and is scheduled to end at 2030 GMT.  Five candidates are running in the opposition contest led by Henrique Capriles, 39, the energetic governor of Miranda state, who has emerged as a frontrunner, 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.