The ceremony was led by National Assembly Speaker Diosdado Cabello in the capital, Caracas.
Hugo Chavez, who died on March 5 after a long battle with cancer, had named Nicolas Maduro as his chosen successor.
However, the main opposition coalition boycotted Nicolas Maduro’s swearing-in, saying that it was unconstitutional.
It argues that – under the constitution – the speaker of the National Assembly should be the one to take over as acting president.
The opposition leader, Henrique Capriles, called the move fraudulent.
After swearing in Nicolas Maduro, Diosdados Cabello said: “Venezuela will follow the route to socialism.”
As acting president, Nicolas Maduro is expected to call elections within 30 days.
Holding a copy of the Venezuelan constitution in his hand, Nicolas Maduro announced at the National Assembly: “I swear in the name of absolute loyalty to Comandante [commander] Hugo Chavez that we will obey and defend this Bolivarian Constitution with the hard hand of the free people.”
Fireworks exploded above Caracas as Nicolas Maduro was sworn in.
Earlier on Friday, Venezuelans paid an emotional farewell to Hugo Chavez.
Nicolas Maduro told mourners that Hugo Chavez, who led Venezuela for 14 years, remained “undefeated, pure, living for all time”.
The former vice-president began the funeral ceremony by presenting Hugo Chavez’s coffin with the sword of Simon Bolivar – the 19th-Century independence leader he claimed as his inspiration.
More than 30 world leaders attended the ceremony, including Cuban President Raul Castro, Iran’s Mahmoud Ahmadinejad and Alexander Lukashenko of Belarus.
A message was read out from Syrian President Bashar al-Assad.
Hugo Chavez, 58, was re-elected for a fourth term as president last October after saying he had recovered from his illness.
He named Nicolas Maduro as his preferred successor following the recurrence of his cancer.
Nicolas Maduro:
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