Maduro, who President Hugo Chavez appointed in October to lead the country while he was in Cuba for a fourth cancer surgery, said the opposition was intent on overthrowing the government.
Speaking in front of the presidential palace to a rally of thousands of red-shirted followers on the day Chavez was supposed to have been inaugurated to a fourth term, Maduro said the opposition, which he labeled the "ultra-right," would stage riots, engage in sabotage and burn cities in an attempt to depose the government, whose leader the opposition has never been able to defeat at the polls -- and this would "fill the country with blood," Maduro said.
"The blood of the Goths runs through them, the blood of aristocrats, sons of wealthy Caracas families, who always reject people of racially mixed ancestry," referring to common Venezuelans, who pride themselves in having a mixed heritage.
The Goths played an important role in the fall of the Roman Empire.
Maduro said the opposition refused to accept Wednesday's Supreme Court's ruling that Chavez, 58, had a right to stay out of the country "indefinitely" due to his ill health.
The court, which in 14 years of Chavez rule never issued a decision against the government, said Chavez could govern in absentia, and be sworn in before the court whenever he returns.
The justices also said there was no need to send a medical team to evaluate Chavez's health, as opponents requested.
"We are evaluating forceful actions," Maduro warned the opposition after questioning its acceptance of the court's decision. "Be careful with your words and actions. Be careful not to take coup-like and destabilizing actions."
Opposition leader Henrique Capriles -- the governor of Miranda, one of the country's most populous states, which includes part of Caracas, the capital -- said he respected the court's decision but said it was politically motivated.
On Twitter he challenged the government, which he said was "paralyzed," to "get to work" Friday and solve Venezuela's pressing problems.
Capriles, who lost to Chavez in October, also noted National Assembly President Diosdado Cabello, who has been in a reported power struggle with Maduro over government control, appeared at the inaugural rally but didn't speak.
"By the way, they didn't let Al Capone talk. What has happened?" Cabello's Twitter comment said.
Cabello, reviled by the opposition, is a former vice president with close ties to Venezuela's military.
Maduro denied he and Cabello were in a power struggle.
"They say Diosdado and I are killing each other," Maduro said in his speech, calling Cabello by his first name as he hugged him.
"But we're killing ourselves for the people and we're killing ourselves for Chavez."
Chavez -- who declared himself fully recovered from his unspecified cancer July 9 -- 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.