Chavez swearing-in postponed
If he dies or steps aside, new elections would be called within 30 days. Before leaving for Havana in December, the president instructed his supporters to back Mr Maduro in that vote if he were unable to continue.
Opposition leaders argue that Congress chief and Chavez ally Diosdado Cabello should take over, as mandated by the constitution if the president's absence is formally declared.
Mr Cabello has ruled that out, saying the president continues to be in charge.
"Who could have believed the opposition would be screaming for Diosdado Cabello to be given the presidency of the republic?" he said during a rambunctious session of Congress. "That's crazy, the opposition is losing it."
Meanwhile opposition deputies accused the Socialist Party of failing to follow Mr Chavez's instructions - a scene that would have been unimaginable before Chavez's prolonged absence.
"President Chavez is the only one among you who has spoken clearly," said opposition leader Julio Borges.
He was drowned out by pro-Chavez deputies clapping and chanting the socialist leader's name and rebuffed by Mr Cabello, who had long been considered a potential successor to Chavez until he was passed over for Mr Maduro.
"It's not my fault you weren't chosen, don't take your frustration out on me," Borges quipped.
Another opposition deputy complained that during the debate a copy of the constitution was thrown across the chamber from the direction of the Socialist Party's deputies.
Mr Chavez's supporters have held near-daily vigils for his recovery, while opposition activists accuse the president's allies of a Cuban-inspired manipulation of the situation.
Mr Maduro has taken over the day-to-day running of the government and looks set to continue in the role past Thursday.
The mustachioed former bus driver lacks Mr Chavez's charisma, but he has sought to imitate the president's style with vituperative attacks on the opposition and televised ribbon-cutting ceremonies.
With the micro-managing Chavez away, major policy decisions in Venezuela, such as a widely expected devaluation of the bolivar currency, appear to be on hold.
Venezuelan bond prices, which had soared in recent weeks on Mr Chavez's health woes, dipped on Monday and yesterday as investors' expectations of a quick government change apparently dimmed.
"The 'regime change' euphoria seems excessive taking into account the unclear legal transition and perhaps, more importantly, the risk that regime change does not allow for policy change," New York-based Jefferies' managing director Siobhan Morden said in a note on the bonds.