"If the president can sign decrees, I call on him to show himself," Capriles said at his gubernatorial inauguration after Chavez's signature appeared in the government's official gazette affirming the appointment of former Vice President Elias Jaua as Venezuela's new foreign minister.
Jaua -- who lost to Capriles in December to be governor of Miranda, one of the country's most populous states -- was appointed foreign minister Tuesday by Vice President Nicolas Maduro.
Capriles called on Chavez -- who hasn't been seen in public since a Dec. 11 surgery in Havana to treat a recurring cancer -- to "speak to Venezuela and say what's going on in this government because what there is in Venezuela today is a 'mis-government.'"
Capriles, 40, has said the government has been paralyzed due to uncertainty over Chavez's health.
"Venezuelans need a government that governs. The time has come to fulfill that and work for all Venezuelans," said Capriles, who lost to 58-year-old Chavez in October presidential elections.
Chavez was to be inaugurated in Caracas, the capital, Jan. 10, but didn't appear. The government said it was due to his health.
In past trips to Cuba for cancer treatment, starting in June 2011, Chavez stayed in the public eye, posting on Twitter, making phone calls to government-run TV stations and once conducting a televised government meeting from Cuba.
The Venezuelan government had no immediate comment on how Chavez's signature appeared in the state publication. Nor did it say why Caracas appeared as the city next to Chavez's signature, even though Chavez has been said by government officials to remain in a Havana hospital.
The Wall Street Journal said Chavez touted technology in July 2011 that he said let him sign documents from anywhere in the world.
"Being in Havana, Moscow, Washington or in Buenos Aires you can sign electronically," the Journal quoted Chavez as saying at the time.
"This of course is totally secure -- you're not going to get false signatures," Chavez said.
Chavez -- who declared himself fully recovered from his unspecified cancer July 9, 2012 -- flew to Havana Dec. 10 for additional cancer surgery. He later developed a severe pulmonary infection that has resulted in breathing difficulties, or a "respiratory insufficiency," the government said Jan. 3.
If Chavez dies or steps down, the Constitution requires elections within 30 days.
Before leaving, Chavez asked Venezuelans to rally behind Maduro if new elections are held.