The Vatican has hit out at what it calls an "anti-clerical, leftwing" campaign against Pope Francis, strongly rejecting accusations concerning his actions during Argentina's brutal military dictatorship.

In a statement at a press briefing on Friday, Federico Lombardi, the pope's spokesman, said the allegations against Jorge Bergoglio, who until Wednesday was the archbishop of Buenos Aires, "must be clearly and firmly denied". "There has never been a concrete or credible accusation in his regard. Argentinian justice interrogated him once … but he was never charged with anything," he said. "He documented his denials of the accusations against him. There are also many declarations that show how Bergoglio tried to protect many people in his time during the military dictatorship. His role is very clearly noted."

The Roman Catholic church has been widely criticised for failing to stand up to the junta that ruled Argentina between 1976 and 1983. Some critics have gone further in their attacks on Bergoglio, claiming he failed to protect two Jesuit priests serving under him who were abducted and tortured for five months at a navy base.

Chief among them is Horacio Verbitsky, an Argentinian journalist, who alleges that Bergoglio – who at the time was not a senior cleric but was the leader of the country's Jesuits – played a double game, helping the pair while at the same time expressing concern about their activities to the military.

Other observers, including human rights activists, have rejected the claims.

The Vatican, for its part, said the accusations came from "anti-clerical, leftwing elements that are used to attacking the church".

Lombardi also pointed out that one of the two priests abducted in 1976, Francisco Jalics, had on Friday released a statement saying he had since celebrated mass with Bergoglio and was reconciled to what happened. In the message, posted on the website of the German Jesuits, Jalics did not explicitly exonerate the new pope, but said he could not "take a position on the role of Father Bergoglio in these events".

He added: "After our liberation I left Argentina. Only years later did we have the chance to speak about what had happened with Father Bergoglio, who had in the meantime been appointed archbishop of Buenos Aires. Afterwards we publicly celebrated mass together and solemnly embraced one another. I am reconciled to what happened and consider the matter closed. I wish Pope Francis God's rich blessings for his office."

The other priest who was kidnapped, Orlando Yorio, died in 2000.

"Bergoglio helped people too," said Rodolfo Yorio, Orlando's brother. "He was two-faced. If the military killed someone, then Bergoglio had nothing to do with it, but if someone was saved, he was the one who saved them."

Others gave a more positive account of Bergoglio's role during the dirty war. In 1973, Alicia Oliveria became the youngest woman to be named a judge in Argentina. But it was a post she did not hold for long. When Argentina's generals staged the 1976 coup, they fired her.

"We were friends already back then and he stood by me when my world became very dark," said Oliveira, who spoke to Bergoglio on the phone only a couple of days before he left for Rome this She said they had not dicussed the possibility that he might be elected Pope. "We don't need to talk about that, besides, he has many enemies here, and you never know who might be listening on the line."

In 1976, when she was suddenly without a job, and living in some fear because of a life-long commitment to human rights causes, Oliveira recalls a moving gesture from Bergoglio: "He sent me a bouquet of roses."

Oliveira said the accusations that Bergoglio failed to speak out against the dictatorship, and withdrew the protection of his Jesuit order from the two priests who were kidnapped, are untrue.

"I personally saw how much he suffered for the priests who were being purused by the dictatorship," said Oliveira. "He talked about it to me continually. At the time we were meeting about twice a week. Before they were kidnapped, he told me how hard he was trying to convince them to stop working in the slums, because it was too dangerous, but they insisted on staying to help the poor. So when they were finally kidnapped, he was devastated and did everything in his power to save them."

On Francis's second full day in office, he met the college of cardinals and reminded them of the duty of older people. "We are in old age. Old age is the seat of wisdom," said Francis, who tripped and stumbled when he greeted a senior cardinal but quickly recovered. "Like good wine that becomes better with age, let us pass on to young people the wisdom of life."

Francis also paid tribute to his predecessor, the emeritus pope, Benedict XVI. "He enriched and invigorated the church with his magisterium, his goodness, guide and faith," he said, pausing and adding: "His humility and his gentleness."

Meanwhile, the Vatican continued to draw attention to aspects of Francis's behaviour that it said showed the new pope's frugality and humility.

Most significantly, Lombardi confirmed that, on the night of his election, the 76-year-old had called the papal nuncio in Argentina and told him to tell the faithful in his home country not to come to his installation mass on Tuesday but to make a donation to the poor.

"He called the ambassador and asked him to tell the bishops that there is no need for them and the faithful to make a long, expensive trip to come to see him but that they could instead offer the money to the poor," said Lombardi.

The spokesman added that Francis had often arrived late at meal times in the Santa Martha house where he was staying with other cardinals and squeezed in wherever there was a space. He added that, on Friday, the new pope had referred to the princes of the church simply as "my brother cardinals".

Francis's penchant for speaking off-the-cuff is causing difficulties in the Vatican's communications department. In his first mass as pope in the Sistine Chapel on Thursday, he had given an unprepared homily, and many of the remarks he made to the cardinals on Friday were also spontaneous. "He added all kinds of things as he was going on," said Father Thomas Rosica, a spokesman, with a wry smile.

Journalists had hoped that the pontiff would bring some of his apparent spontaneity to a meeting with them on Saturday, but were disappointed to learn that the audience would involve no opportunity to ask questions.