Vice-President Nicolas Maduro said the Supreme Court could swear in Hugo Chavez at a later date.
Nicolas Maduro dismissed opposition calls for new elections should Hugo Chavez not attend.
President Hugo Chavez is in Cuba struggling to recover from his latest round of surgery to treat cancer.
He has not been seen in public since the operation more than three weeks ago.
Observers have different interpretations of what it would mean if Hugo Chavez misses his inauguration on Thursday.
Some in the opposition have said that if Hugo Chavez is still in Cuba, power should pass to the speaker of parliament, and new elections should be held within 30 days.
But Nicolas Maduro said Thursday was not a fixed deadline, and that there was no reason to declare Hugo Chavez’s “absolute absence” from office.
“The formality of his swearing-in can be resolved in the Supreme Court,” he said.
“The president right now is president,” he said, waving a pocket-sized copy of the constitution.
“Don’t mess with the people. Respect democracy.”
The head of the main opposition coalition, Ramon Guillermo Aveledo, said the government “doesn’t want to admit that the president is absent”.
“The official version of what is happening is unsustainable,” he told reporters.
Officials have said that Hugo Chavez, 58, has suffered from complications brought on by a severe lung infection that developed after his latest surgery.
Nicolas Maduro said the president had “a right to rest and tranquility, and to recuperate”.
“We will have the commander well again.”
Nicolas Maduro and National Assembly Head Diosdado Cabello visited Hugo Chavez in Cuba earlier in the week, along with several other dignitaries.
The vice-president said President Hugo Chavez was “conscious” and had gripped his hand firmly as they discussed Venezuelan politics.
Nicolas Maduro and Diosdado Cabello dismissed rumors of a split in the governing socialist movement, after their return from Cuba.
“We’re more unified than ever,” said Nicolas Maduro, who Hugo Chavez recently named as his preferred successor.
“We swore in front of Commander Chavez that we’ll be united at the side of our people,” he said.
The National Assembly is due to meet on Saturday to elect its leadership, with Diosdado Cabello expected to be re-elected.
He has appealed to supporters of Hugo Chavez to rally outside parliament during the session.
“If the opposition thinks it will find a space in the National Assembly to conspire against the people, it’s mistaken once again,” Diosdado Cabello said on Twitter.
“It will be defeated.”
Hugo Chavez was re-elected to a fourth term as president in October.
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