In a wide-ranging speech, Maduro promised to defeat US sanctions and take steps to correct the course of an economy in ruins, including seeking OPEC support to nearly double its oil output.
Wearing a ceremonial sash in the Venezuelan colors, Maduro swore "to respect and enforce the constitution and lead all revolutionary changes" in a ceremony before the Constituent Assembly, which he set up himself last year and stacked with his supporters.
The socialist leader said those changes should lead Venezuela to "the peace, prosperity and happiness of our people."
Striking a rare conciliatory note, Maduro admitted a fresh take on Venezuela's problems was needed, in a lengthy speech before the military and government hierarchy.
"We need a profound rectification, we have to do things anew and better, we are not doing things well and we have to change this country," he acknowledged.
He admitted that tighter sanctions imposed by Washington after his re-election will bring more difficulties to the oil producer, not least because they will prevent "necessary imports."
"I cannot deceive anyone, they are going to create serious difficulties for us, painful difficulties, that we will face gradually – we will defeat them," he said.
Venezuela is going through the worst economic crisis in its history.
Hyperinflation has crippled the country, leading to shortages of food and medicine. Hundreds of thousands of people have fled to escape the growing deprivation.
Venezuela's constitution states that the president must be sworn in before parliament, where the opposition holds the majority and which has in practice been replaced by the Constituent Assembly.
The parliament was declared in contempt by the Supreme Court two years ago, and consequently its decisions are now considered null and void.
Under the constitution, the inauguration of Maduro's new term was to be held next January. Prior to the swearing-in, the Assembly approved by a show of hands a decree clarifying the new mandate would begin on January 10 even if Maduro was to be sworn in immediately.
The opposition Democratic Unity Roundtable (MUD) coalition called Thursday's ceremony a "farce."
Oil output increase
Despite sitting on the world's biggest oil reserves, Venezuela has seen a catastrophic fall in oil production to a 30-year low of just 1.5 million barrels a day, largely due to its lack of investment in infrastructure.
Turning to Venezuela's oil minister, General Manuel Quevedo, who was attending the ceremony, Maduro said Venezuela would have to increase its output by one million barrels per day.
"This year we have to increase one million barrels. If we have to ask for support from OPEC, ask for it, Major General – support from Russia, China, the Arab countries, let's ask!"
Oil accounts for 96% of Venezuela's foreign revenues. While official OPEC figures put production at 1.5 million barrels a day in April, sources cited by the cartel claim the level fell even lower, to 1.4 million barrels per day.
Maduro also promised to work for reconciliation and offered to free political opponents who have not committed serious crimes, to "overcome the wounds" of the protests against him, which have left some 200 dead since 2014.
Later in the day, he attended an event at the defense ministry where he received an "reaffirmation of loyalty" from the armed forces' high command.
During the ceremony, he announced the arrest of an unspecified number of troops for allegedly conspiring against the elections, in an operation "financed and directed from Colombia and promoted by the United States government."
The 55-year-old former bus driver's re-election Sunday, May 20, in a vote boycotted by the main opposition was widely condemned by the international community, including the United States, which denounced it as a "sham."
The European Union said it was also weighing new sanctions after the election was marred by "irregularities" and failed to meet international standards.
Maduro, whose term as president now extends to 2025, leads a government facing increasing international isolation, with the United States, the European Union and the 14 countries of the Lima Group – which includes Argentina, Brazil and Canada – refusing to recognize the election result. – Rappler.com