The ceremony was led by National Assembly Speaker Diosdado Cabello in the capital, Caracas.
Mr Chavez, who died on 5 March after a long battle with cancer, had named Mr Maduro as his chosen successor.
However, the main opposition coalition boycotted Mr 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.
Nicolas Maduro
Born in Caracas in 1962
Former bus driver who began political career as a trade unionist
Campaigned for Hugo Chavez's release from prison in 1994
Speaker of the National Assembly from 2005-2006
2006 becomes foreign minister
2012 appointed vice-president
Has long-standing ties with Cuba where he trained as a union organiser
Described as a wily operator and a skilled negotiator
After swearing in Mr Maduro, Mr Cabello said: "Venezuela will follow the route to socialism."
As acting president, Mr Maduro is expected to call elections within 30 days.
Holding a copy of the Venezuelan constitution in his hand, Mr 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 Maduro was sworn in.
Earlier on Friday, Venezuelans paid an emotional farewell to Hugo Chavez.
Mr Maduro told mourners that Mr Chavez, who led Venezuela for 14 years, remained "undefeated, pure, living for all time".
The former vice-president began the funeral ceremony by presenting Mr 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.
Mr Chavez, 58, was re-elected for a fourth term as president last October after saying he had recovered from his illness.
He named Mr Maduro as his preferred successor following the recurrence of his cancer.
