Chavez is in Cuba undergoing cancer treatment that has involved four surgical procedures so far.
Vice-President Nicolas Maduro told the National Assembly in Caracas the decision to appoint Elias Jaua as Venezuela's new foreign minister had come from Chavez.
The opposition challenged the announcement, which came during an address characterized as a state of the nation message.
Both the ministerial appointment and address to the nation were responsibilities of the president and should not have devolved to Maduro, the opposition said.
The opposition's challenge was dismissed by Maduro and there was no indication that any of the senior aides disagreed with his assertion the constitution had not been flouted.
The National Assembly session was led by Speaker Diosdado Cabello and senior officials present at the meeting included Supreme Tribunal of Justice President Luisa Estella Morales, Republican Moral Council President Adelina Gonzalez and National Electoral Council President Tibisay Lucena, Venezuelan official media said.
The opposition fears Chavez's absence at key events and increased representation of the president by Maduro is leading to a de facto continuity that flouts the constitution. Chavez aides deny the allegation.
Maduro's report to the nation on the economic and political conditions in Venezuela in fiscal 2012 was a function of Chavez that the vice president presented to the National Assembly amid protests, including a walkout by at least 12 members.
Chavez aides explained the appointment of Elias Jaua was necessary because Maduro could not continue holding the portfolio while acting daily on behalf of Chavez. Maduro had kept the foreign minister's portfolio after his appointment as vice president in October 2012.
Opposition critics have been calling for postponement of all functions that require the presence of Chavez.
Opposition leader Henrique Capriles, who faced Chavez in last year's presidential election, called for a review and further discussion on the reshuffle as it was unclear how much much of the decision-making had come from Chavez and how much of it was the handiwork of Maduro.
Rafael Ramirez, Venezuela's energy minister and head of the state-owned PDVSA petroleum company, also weighed in with support for Maduro and accused Venezuela's right-wing opposition of fanning a controversy and orchestrating the walkout.
Chavez aides insist the president remains in charge but are unable to specify when he will return from Cuba.
Cabello defended Maduro, saying the vice president "didn't come to take the place of the president. He came to bring the documents ... under instructions from the president," MercoPress reported.
In Washington, U.S. Assistant Secretary of State for Western Hemisphere Affairs Roberta S. Jacobson said the United States is waiting for the situation in the Latin American country to clear up.
"It is a difficult time for Venezuelans," Jackson said, recalling that U.S. President Back Obama has expressed wishes for a quick recovery of Chavez.
Jacobson has been in telephone conversations with Maduro in an indication of what analysts say may be a thaw in relations between Caracas and Washington.
Despite continuing strain in bilateral U.S.-Venezuela relations Venezuela remains one of the largest oil suppliers to the United States, after Canada and Saudi Arabia.