CARACAS - Industrial engineer Maria Corina Machado declared victory overnight in the Venezuelan opposition's presidential primary, after she tallied a huge portion of votes with the count just over a quarter finished.

The opposition held the contest to choose a unity candidate to face President Nicolas Maduro in his probable re-election bid next year, amid pledges by the United States to roll back sanctions relief if the government fails to lift bans preventing some opposition figures from holding office.

Machado, who has pledged to privatize state oil company PDVSA if elected president, was tallying 93% of the vote, with more than 26% of ballot boxes counted, the primary's organizing commission said around midnight on Sunday (0400 GMT on Monday).

The count - delayed by a server blockage - was expected to continue on Monday. It was unclear when the next results update would be given.

Participation in the vote, organized without government help, was more than double what had been expected in some states, despite the relocation of polling places, long lines, and the lack of gasoline and public transport in some areas.

Machado's nearest rival, former lawmakers Carlos Prosperi, had 4.75% of the vote. Machado, 56, had led her rivals by some 40 points in polls.

Her ability to run in the general election remains uncertain as she is still barred from public office over her support of the sanctions on Maduro's government.

The opposition and government last week signed an election deal allowing each side to choose its candidate according to internal rules but not retracting the disqualifications.

The United States, which broadly eased sanctions on Venezuelan oil and gas and bonds in response to the deal, has said Maduro has until the end of November to begin rescinding the bans against the opposition and releasing political prisoners and "wrongfully detained" Americans.

Though five people were released, the Maduro government said last week that those with disqualifications cannot run in the 2024 contest.

The opposition, which says the disqualifications are unlawful, has been reticent about what it would do if Machado wins the primary but is unable to compete in 2024.

Machado has said she could pressure the electoral authorities to let her register, while others have argued a substitute will be necessary.

(Reporting by Vivian Sequera and Mayela Armas; Writing by Julia Symmes Cobb; Editing by Tom Hogue)