The Vatican on Friday rejected claims that Pope Francis failed to do enough to protect two priests kidnapped and tortured during Argentina's military rule, saying he had in fact helped save lives.
The first pope to hail from Latin America has been criticised by leftists for his actions during Argentina's "Dirty War" in which 30,000 people died or disappeared from 1976 to 1983.
Jorge Mario Bergoglio, now Pope Francis, was the head of the Jesuit order in Argentina at the time. His role in the arrest of two young Jesuits, Orlando Yorio and Francisco Jalics, who were taken to a notorious torture centre by the brutal right-wing junta, has come under intense scrutiny.
Vatican spokesman Federico Lombardi said: "There has never been a credible, concrete accusation against him. The Argentinian justice system... has never charged him with anything."
He said the campaign against Bergoglio was "well known" but claimed it was defamatory and aimed at discrediting the Catholic Church.
"The accusations come from parts of the anti-clerical left to attack the Church and must be denied," said Lombardi, insisting that Bergoglio "did a lot to protect people during the dictatorship" when he was not yet a bishop.
Bergoglio himself has always denied any involvement in the case, and even says he intervened with the head of the junta, Jorge Videla, to beg for the Jesuits to be freed. The two priests were released after five months.
-- 'Let us not give in to pessimism' --
The newly elected pontiff, who is also the first Jesuit pope, earlier urged the troubled Catholic Church that he has inherited not to succumb to "pessimism" and to find new ways of spreading the faith.
"Let us not give in to pessimism, to that bitterness that the devil offers us every day," Francis told an audience of the world's cardinals on his third day in office.
In a reference to the declining number of worshippers in many parts of the world, he urged the cardinals to find "the courage to persevere and also to find new ways to bring evangelisation to the ends of the earth".
Francis, 76, said he and the cardinals were "elderly", but old age brought wisdom.
"Let us give this wisdom to young people like good wine that gets better over the years," he told them.
Francis hailed his predecessor Benedict XVI's historic resignation as a "courageous and humble act".
Benedict, who last month became the first pope to stand down for 700 years, "lit a flame in the depth of our hearts that will continue to burn", he said.
Francis wore white papal vestments but also plain black shoes, not the red shoes favoured by his German predecessor, for the address in the ornate 16th-century Clementine Hall in the Apostolic Palace at the Vatican.
He has signalled he will lead a simpler papacy, stripped of the fineries enjoyed by his predecessors and emphasising the importance of pastoral care.
On Friday, Francis visited Argentinian cardinal Jorge Mejia in a local Rome hospital, a day after the 90-year-old suffered a heart attack.
After turning up announced, he spent 20 minutes with Mejia before blessing the hands of the cardiologist treating him.
He chatted warmly with staff and also blessed patients in intensive care before going to the hospital's chapel to pray.
The new pontiff's inauguration mass will take place on Tuesday -- the Feast of St Joseph, the patron saint of the universal church.
He has called on the faithful in his native Argentina not to fly in for the mass but rather give the money the trip would have cost to charity. Nonetheless, heads of state from all over the world are expected to be present.
Francis is also due to meet his predecessor, who has withdrawn to the papal summer residence in Castel Gandolfo, in the coming days.
The surprise election of the son of an Italian emigrant railway worker, who was considered a rank outsider before the cardinals began their confidential deliberations, has sparked hope for change in the Church.
Francis fans have already begun snapping up rosary beads and postcards with his face on them at souvenir shops in the Vatican, and New York's Bice restaurant has created a new dish -- barbecued steak pasta -- in homage to the pope's Argentinian and Italian roots.
His election is being seen as a nod to the Church's power in Latin America, which is home to 40 percent of the world's Catholics.
In Europe, the Church's traditional power base, it is ageing and declining.
As archbishop of Buenos Aires, he lived in a modest apartment rather than the official residence, and he has already made his mark in Rome with his informal style.
Following his election Francis chose to ride in a minibus with his fellow cardinals rather than the papal limousine -- and he later went to pack his own bags at the lodgings where he was living before the conclave to move in to the Vatican.
The Vatican said staff at the clergy house in central Rome knew him well from previous visits and were overcome with emotion when he greeted them by name and asked after their families.