Venezuela's Democratic Unity bloc of opposition parties suggested Saturday it was time for the government to declare the president temporarily absent from power.
"They are trying to hide what every day is a fact: The government does not want to recognize that there is a temporary absence of the president from his duties," the bloc said in a statement.
On Friday morning, Maduro read a New Year message from Chavez to Venezuelan troops, though it was unclear when the president composed it.
"I have had to battle again for my health," Chavez said in the message. He expressed "complete faith in the commitment and loyalty that the revolutionary armed forces are showing me in this very complicated and difficult moment."
A group of opposition candidates demanded Friday that Maduro provide an official medical report on Chavez's health. Lawmaker Dinorah Figuera said the country needs "a medical report from those who are responsible for the diagnosis, evaluation and treatment of the president."
"The Venezuelan people deserve official and institutional information," Figuera told Venezuelan media.
A legal fight is brewing over what should happen if Chavez, who was re-elected in October, cannot return in time for the inauguration before the National Assembly.
National Assembly Diosdado Cabello insisted Monday that Venezuela's constitution allows the president to take the oath before the Supreme Court at any time if he cannot do it before the legislature on Jan. 10.
Opposition leaders argue the constitution requires that new elections be held within 30 days if Chavez cannot take office Jan. 10. They have criticized the confusion over the inauguration as the latest example of the Chavez government's disdain for democratic rule of law and have demanded clarity on whether the president is fit to govern.
Associated Press writers Fabiola Sanchez and Alexandra Olson in Caracas, Venezuela, contributed to this story.