Venezuelan National Assembly is due to begin its new session as the ill-health of President Hugo Chavez casts doubt over his inauguration on January 10.
Current leader Diosdado Cabello is due to be re-elected head of the assembly – dominated by Hugo Chavez’s supporters.
President Hugo Chavez is in Cuba struggling to recover from his latest round of surgery to treat cancer.
Opposition leaders are calling for new elections if he cannot be sworn in for his new six-year term on Thursday.
But Vice-President Nicolas Maduro has said the Supreme Court can swear in Hugo Chavez at a later date.
The normally routine opening of the National Assembly has been given added significance because of Hugo Chavez’s absence.
Under the constitution, the head of the assembly must lead the country if new elections are called.
Analysts say Diosdado Cabello is seen as a political rival to Nicolas Maduro, whom Hugo Chavez has named as his successor.
However, both men have vowed to maintain unity in the governing United Socialist Party of Venezuela (PSUV).
Both visited Hugo Chavez in Cuba earlier in the week, along with several other dignitaries.
Diosdado Cabello called on Hugo Chavez supporters to rally outside parliament on Saturday “to exhort revolutionary unity and head off the campaign of rumors”.
“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.”
Experts have different interpretations of what it would mean if Hugo Chavez misses his inauguration.
Some in the opposition say that if Hugo Chavez is still in Cuba, power should pass to the head of the National Assembly and new elections should be held within 30 days.
But Nicolas Maduro has insisted that Thursday is 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.”
Hugo Chavez – who was re-elected for a fourth term in October – has not been seen in public since his latest round of surgery more than three weeks ago.
Information Minister Ernesto Villegas said on Thursday that the president had suffered complications due to a lung infection and had a “respiratory insufficiency”.