Late President Hugo Chavez designated Maduro as his successor before he died Tuesday of cancer. Maduro had been Chavez's vice president.
The country's 1999 constitution says the National Assembly speaker becomes interim president in the event of a president-elect's death or inability to be sworn in. The constitution also says a presidential election should be called within 30 days.
Maduro has been picked as the presidential candidate of Chavez's socialist party.
Opposition leader Angel Medina said earlier Friday that the 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.
He also echoed accusations he made shortly before Chavez's death that the U.S. had caused the fatal cancer. On Friday night, he said Chavez's cancer was scientifically ``strange.''
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. (AP)