Vice President Nicolas Maduro broke the news in a letter to National Assembly President Diosdado Cabello, saying on the recommendation of Chavez's medical team, his recovery process "should be extended beyond Jan. 10" and for that reason he won't be able to attend Thursday's scheduled inauguration.
Maduro said Chavez was invoking a provision in the constitution allowing him to be sworn in before the Supreme Court at a "later date." Cabello announced he had received the request during a legislative session.
Tensions between the government and opposition have been building in a constitutional dispute over whether the ailing president's swearing-in can legally be postponed. The president underwent his fourth cancer-related surgery in Cuba last month and hasn't spoken publicly in a month.
Opposition leader Henrique Capriles said earlier Tuesday that Chavez's current term constitutionally ends Thursday and that the Supreme Court should rule in the matter.
Other opposition leaders have argued that the inauguration cannot legally be put off and that the National Assembly president should take over as interim president if Chavez hasn't returned from Cuba on inauguration day.
"The Supreme Court has to take a position on what the text of the constitution says," said Capriles, who lost to Chavez in presidential elections three months ago. "There is no monarchy here, and we aren't in Cuba."
However, Capriles said he saw no reason to bring a formal challenge to the Supreme Court because it was obliged to issue a ruling on the dispute.
While leaders of both pro- and anti-Chavez camps say they don't expect violence to break out Thursday, the dispute could lead to opposition questions about the legitimacy of government officials serving past the scheduled inauguration date.
The Venezuelan Constitution says the presidential oath should be taken before lawmakers in the National Assembly on Jan. 10 but adds that the president may also take the oath before the Supreme Court if he's unable to be sworn in before the assembly. Government officials argue that clause does not explicitly mention a date, though opponents say it clearly refers to the Jan. 10 deadline.
Maduro said in his letter that Chavez's condition was an "irrefutable unexpected reason" that made it impossible for Chavez to attend the scheduled inauguration.
The announcement set off an impassioned debate in the National Assembly. Opposition lawmaker Omar Barboza dismissed Maduro's proposal and urged Chavez's allies to accept Cabello as interim president while Chavez recovers, saying that was to avoid an "institutional crisis."
Barboza said it's clear that a "temporary absence" should now be declared, which would give the president 90 days to recover, which could be renewed for another 90 days.
Some lawmakers called for a medical team to be formed to determine Chavez's state. Some also questioned why the letter was signed by the vice president rather than Chavez himself.
"Who's governing Venezuela? In Venezuela, Havana is governing, and that's the problem we have," opposition congressman Julio Borges said during the debate.
Barboza said that if Chavez's allies continue with their stance, the opposition will be forced to "convoke the people of Venezuela to re-establish the validity of the constitution." He didn't elaborate.
Maduro has called the swearing-in a "formality" and said the opposition is erroneously interpreting the constitution. Chavez has said that if he's unable to continue on as president, Maduro should take his place and run in an election to replace him.
Capriles noted, however, that Maduro "wasn't elected" to continue leading a government in Chavez's absence into a new term. "If Maduro wants to be president, it's not through that way," Capriles said.
He added Tuesday that he has spoken with members of the military, and that they have told him "we are with the constitution."
"The Armed Force also has a role to play to play here ... of respecting the constitution," Capriles said, without elaborating, while using the military's formal name.
Jorge Rodriguez, a Caracas district mayor and campaign manager in recent elections, accused the opposition of fomenting a "conspiracy" against Chavez's government. He insisted that Chavez remains president despite his health problems and pointed out that the National Assembly had granted the president permission to travel to Cuba for his operation.
The government had said on Monday that Chavez was in a "stable situation" in a Cuban hospital receiving treatment due to a severe respiratory infection. The government says he's coping with "respiratory deficiency," but hasn't said how severe it is.
Government officials called for Chavez's supporters to gather at the presidential palace on Thursday, and said some Latin American heads of state plan to attend.
Bolivia's government confirmed that President Evo Morales would travel to Caracas.
Uruguayan President Jose Mujica also plans to fly to Caracas on Wednesday morning, Deputy Foreign Relations Minister Roberto Conde told local station Radio Uruguay on Tuesday.
Capriles urged Latin American leaders not to visit, asking them to instead demand that the Venezuelan Constitution be upheld. He said presidents of other countries such as Argentina, Brazil and Colombia should also take such a stance.
The opposition leader said he won't call for protests on inauguration day.
"What I won't do is put people to fight against people. Don't count on us," Capriles told reporters. "Our country doesn't need hate. Our country doesn't need fights."
Opposition coalition leader Ramon Guillermo Aveledo wrote to Organization of American States Secretary-General Jose Miguel Insulza on Monday explaining the opposition's concerns. Aveledo said in the letter that if Chavez doesn't appear on inauguration day and there is no declaration of a temporary absence of the president, "a serious violation of the constitutional order will have been committed."
Associated Press writers Christopher Toothaker and Ian James contributed to this report.
AP Interactive: http://hosted.ap.org/interactives/2012/venezuela/