The Vatican on Friday rejected claims that Pope Francis failed to do enough to protect two priests kidnapped and tortured by Argentina's military junta and said he had in fact helped save lives.
Jorge Mario Bergoglio, the first pope to hail from Latin America, has been criticised by leftist critics for his actions during Argentina's "Dirty War" in which 30,000 people died or disappeared from 1976 to 1983.
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 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 them to be freed. The two men were released after five months.
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," he 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 they 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 the cardinals.
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, had "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 more simple papacy, stripped of the fineries enjoyed by his predecessors.
On Thursday, he gave a stark warning that the Church, wracked by scandal and Vatican infighting, risked becoming just another charitable organisation if it strayed from its true mission.
His inauguration mass will take place on Tuesday -- a significant date in the Catholic calendar because it is the Feast of St Joseph, the patron saint of the universal church.
The new pontiff 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.
His elevation 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, its traditional power base, it is ageing and declining.
Projecting an image as a simple man of the people, the pope chose to name himself after St Francis of Assisi, the 13th century saint who shunned the riches of his family to devote himself to God and the poor.
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.
The Vatican revealed that following his election Francis had chosen to ride in a minibus with his fellow cardinals rather than the papal limousine. He also returned to his lodgings to pay his own bill.
With health always an issue surrounding new popes -- John Paul I only lived for just 33 days after he was elected in 1978 -- the Vatican confirmed that Francis had part of a lung removed as a boy, but insisted that he is in good health.
The pope faces the challenge of stamping his authority on the Vatican machinery and trying to bring back people in the West who are turning their back on Catholicism.
He must also confront the continuing fallout from the sexual abuse of children by paedophile priests stretching back decades.