Venezuela's main opposition coalition said it was prevented from registering its presidential candidate to run against President Nicolas Maduro in July, after he made his re-election bid official.
Once the midnight nomination deadline lapsed, the Democratic Unitary Platform (PUD) said it had not been able to officially register its candidate.
"We have been working all day ... trying to exercise our constitutional right to nominate our candidate. This was not possible," coalition official Omar Barboza said in a video released by PUD early Tuesday.
Maduro, 61, formalized his own run for the presidency with great fanfare on Monday, with thousands turning out to rally behind him and the ruling United Socialist Party of Venezuela.
"I was moved by so much generosity and recognition on the part of the people for this humble man from the neighborhoods of Caracas, this humble worker," said Maduro, carrying an illustration of his mentor, revolutionary leader Hugo Chavez.
The former bus driver is seeking to extend his turbulent time in power with a third six-year term, amid rising concerns over his slide into authoritarianism and crackdown on the opposition.
Wearing a jacket in the red, yellow and blue of the Venezuelan flag, he called on his supporters to rally behind him, as the crowd chanted his name.
"Nicolas is the hope, he is the continuity of a project that Commander Hugo Chavez started," Pedro Mata, 52, told AFP at the rally.
However, opinion surveys show that Maduro is not the favorite at the July 28 polls.
Opposition leader Maria Corina Machado, 56, won an opposition primary vote last year hands down, and some surveys put her support at about 72 percent.
She was declared ineligible however, and banned from public office for 15 years by courts loyal to Maduro on charges of corruption widely dismissed as spurious, and for supporting Western sanctions against the government.
Machado nevertheless kept campaigning, and on Friday tapped 80-year-old university professor Corina Yoris as her stand-in.
The opposition coalition said its bid to register Yoris proved unsuccessful.
One of PUD's coalition members, Un Nuevo Tiempo (UNT), was separately able to register a candidate, Zulia state Governor Rosales Manuel, according to an announcement from the National Electoral Council (CNE).
"They did it by automated means," CNE president Elvis Amoroso said.
Rosales, 71, contested the 2006 elections, when he was defeated by Chavez.
Just before the registration clock ran out, PUD said Monday night it had not yet received the necessary access codes to nominate Yoris on the CNE website.
"The system is completely closed," Yoris told a press conference.
"My rights as a Venezuelan citizen are being violated," she said.
She said that her team had gone in person to the CNE to deliver a letter requesting a three-day extension to the period to nominate candidates, but had been unable to do so.
In a joint statement, Argentina, Costa Rica, Ecuador, Guatemala, Paraguay, Peru and Uruguay expressed concern over Yoris's inability to register.
At least 10 opposition candidates did manage to register, however most are considered aligned with Maduro's government.
The final list of presidential candidates will be published at the end of April.
UN chief Antonio Guterres last week warned against interference in the election.
Seven of Machado's party and campaign officials have been arrested, and warrants have been issued for several more, all accused of seeking to destabilize the country.
Six years later Maduro, 61, is still firmly in charge of the oil-rich nation after his rival's government collapsed and the war in Ukraine choked energy supplies and shifted global priorities.
