Maduro has been picked as the presidential candidate of Chavez's socialist party.
Opposition leader Angel Medina said earlier Friday that opposition would boycott the swearing-in ceremony.
Stray fireworks exploded above the capital of Caracas as soon as Maduro was sworn in as president.
Both Maduro and National Assembly President Diosdado Cabello pledged to follow Chavez's example and push his socialist-inspired agenda.
"I swear by the most absolute loyalty to comrade Hugo Chavez that we will fulfill and see that it's fulfilled the constitution ... with the iron fist of a people ready to be free," Maduro said.
After Cabello swore in Maduro, the National Assembly president said, "Venezuela will follow the route to socialism."
Opposition leader Henrique Capriles said Maduro used Chavez's funeral earlier in the day to campaign for the presidency, in violation of the constitution. Capriles is widely expected to run against Maduro in the coming vote.
Venezuela's opposition leader calls Maduro a liar
Venezuela's opposition leader said Friday that Vice President Nicolas Maduro had "shamelessly" lied to the country and accused him of using Hugo Chavez's funeral to campaign for the presidency.
Henrique Capriles also said in a speech that the opposition had asked to attend Chavez's funeral but was told "better that you don't come."
Capriles spoke condescendingly of Maduro, calling him "boy."
Capriles lost the Oct. 7 presidential election to Chavez. He said he decided to speak Friday to object to the ruling earlier in the day by the country's highest court that Maduro had become acting president the moment Chavez died.
The government announced Thursday night that Maduro would be sworn in Friday night. He will be the governing socialists' presidential candidate in the vote to replace Chavez.
Capriles said he had withheld criticism since Chavez's death out of respect, but could no longer hold his tongue at what he saw as a power grab.
"I tell you clearly, Nicolas, I am not going to speak of the times you lied to the country, shamelessly," Capriles said. "The people have not voted for you, boy."