The president of the National Assembly Diosdado Cabello read a letter from Vice President Nicolas Maduro during a regular session, informing the parliament of Chavez's inability to take the oath of office on that day.

"The president has requested to inform that according to the recommendations of his medical team ... the post-operative process of recuperation must extend beyond Jan. 10," the letter said.

For that reason, Chavez "will not be able to appear before the National Assembly on that date," the letter added.

Chavez, reelected to another six-year term in October, was to be sworn in at the assembly.

Though it appeared unlikely that he would be well enough to attend the ceremony, especially after contracting a lung infection during recovery, there was no confirmation until now.

His aides have said the ceremony, being a mere formality, can be postponed, but the country's opposition has opposed the move.

The letter goes on to say given Chavez's state of health, the government will "invoke Article 231 of the ... Constitution, with the aim of formalizing at a later date the corresponding swearing in before the Supreme Court of Justice."

To that end, the National Assembly session Tuesday was expected to discuss granting Chavez the permission to miss the inauguration Thursday.

The opposition maintains that failure to take the oath of office on the designated day will lead to a power vacuum that can only be addressed by naming the head of the National Assembly, in this case Cabello, as interim leader.

To counter pressure from the opposition, the government has called for a mass rally Thursday in support of Chavez, and has invited the heads of state of "friendly" countries to attend.

The presidents of Bolivia and Uruguay have so far indicated they will be in Venezuela's capital Caracas Thursday to participate in the massive show of support for the ailing Chavez.

"The rally's goal is first to confirm (Chavez's) victory in the elections held in Venezuela, and second to express the solidarity of Latin American leaders, including Evo Morales, with Hugo Chavez, as well as with the Venezuelan government and people," Bolivia's Communication Minister Amanda Davila said Tuesday.

Uruguay's Senate, meanwhile, authorized President Jose Mujica's travel to Venezuela to participate in the rally for Chavez.

During the proceedings, Uruguayan Vice President Danilo Astori, who is also president of the Senate, said Venezuela's Foreign Ministry invited Mujica as the leader of his country and current president of the Southern Common Market (Mercosur) trade bloc, which Venezuela became a full member of in 2012.

"Our work is to support Venezuela's government and people," Mujica said, according to La Republica daily.

Reelected to another six-year term in October, Chavez was to have been sworn in at the National Assembly Thursday, but remains hospitalized in Havana, Cuba after undergoing cancer surgery for the fourth time in December.