Popular Will party opposition figure Freddy Superlano and Machado have already formally registered, while Capriles said he plans to do so on Saturday, the final registration day.
The fractured opposition, which runs a parallel legislature recognized by the United States and controls the country's foreign assets, was already facing widespread voter apathy.
Now it has a new challenge - the resignation this month of members of the electoral council, which has obliged the opposition to hold the vote without state funding or help.
Replacing the electoral council - which the opposition has long alleged is biased towards the ruling party, but which included two opposition members - could take more than two months.
That leaves little time for planning before the Oct. 22 vote, forcing the opposition to organize without the council, Jesus Maria Casal, the head of the opposition primary committee has said.
"The real truth is that this election will be very complicated," said pollster Saul Cabrera. "It's not just the capacity to organize, but that everything has a cost in dollars."
The opposition has not confirmed how much the primary could cost, but analysts estimate printing and transporting ballots, giving snacks and transport to polling volunteers and advertising voting locations could cost between $5 million and $7 million.
Former legislators Roberto Enriquez, Tamara Adrian, Cesar Perez Vivas and Delsa Solorzano and former governor Andres Velasquez have already registered.
Manuel Rosales, governor of oil state Zulia, ex-lawmaker Carlos Prosperi, and economist and politician Andres Caleca are also expected to register.
Benjamin Rausseo, a comedian known as 'El Conde', will not participate in the primary, running instead as an independent.
The primary may also face roadblocks put in place by the ruling party, said Benigno Alarcon, director of the Center for Political and Government Studies at Andres Bello Catholic University in Caracas.
"The government will do all that is humanly possible so there isn't a primary and if there is, that it goes very badly," he said. "For Maduro the ideal is for several opposition candidates to run (in the presidential race), taking votes from each other."
The communications ministry and electoral council did not respond to questions about the primary.
Up to 3 million of Venezuela's 21 million eligible voters could participate in the primary, Alarcon said.
(Reporting by Vivian Sequera; Writing by Julia Symmes Cobb; Editing by Daniel Wallis)
By Vivian Sequera
