According to TV2000, a Catholic television channel, the act was carried out in St Peter's Square after Mass on Sunday.

Smiling broadly, the Pope initially shook the man's hand, but the South American pontiff's expression changed dramatically after a priest from the Legionaries of Christ, a conservative order, leaned in close and spoke a few words to him.

With a more serious expression on his face, Francis placed both hands on the man's head for 15 seconds.

The pilgrim, said to be a 43-year old married man from Mexico called Angelo, then convulsed briefly and emitted a long sigh. His body went limp and his mouth dropped open.

"Exorcists who have seen the footage have no doubt - this was a prayer for liberation from Evil, an actual exorcism," said TV2000, which is owned by the Italian Bishops Conference.

The station gathered a panel of clergy specialising in exorcisms who scrutinised the footage and concluded that the pope had performed an exorcism.

The Vatican downplayed the incident, although it used ambiguous language that did not deny altogether that Francis had tried to rid the man of evil.

"The Holy Father did not intend to carry out any exorcism," said Father Federico Lombardi, the Vatican spokesman.

"Instead, as he often does for sick and suffering people, he simply intended to pray for a person who was presented to him."

Leading exorcists insisted that the Pope had indeed taken on the forces of evil.

"The Pope is also the Bishop of Rome, and like any bishop he is also an exorcist," Father Gabriele Amorth, the Catholic Church's best known exorcist and the head of the International Association of Exorcists, told La Repubblica newspaper.

"It was a real exorcism," he said. "If the Vatican has denied this, it shows that they understand nothing", said Father Amorth who claimed that the Mexican was "possessed by four demons".

The Rev. Giulio Maspero, a Rome-based systematic theologian who has witnessed or participated in more than a dozen exorcisms, said he was certain that Francis' prayer on Sunday was either a full-fledged exorcism or a prayer to "liberate" the young man from a demonic possession. He noted that the placement of the pope's hands on the man's head was the "typical position" for an exorcist to use.

"When you witness something like that - for me it was shocking - I could feel the power of prayer," he said in a phone interview, speaking of his own experiences.

Fr Maspero said it was particularly symbolic that Francis' purported exorcism occurred on Pentecost, an important feast day for the church when the faithful believe Jesus' apostles received the fullness of the Holy Spirit.

"The Holy Spirit is connected to the exorcism because ... it is the manifestation of how God is present among us and in our world," he said.

There was now, more than ever, a need for exorcists to combat people possessed by "sorcerers" and "Satanists", Father Amorth said.

"We live in an age in which God has been forgotten. And wherever God is not present, the Devil reigns."

He acknowledged that many people, even Catholics, regarded exorcism as mumbo-jumbo but insisted they were mistaken. "Those who don't believe should read the Gospels. Jesus continually performed exorcisms.

"Today, unfortunately, bishops appoint too few exorcists. We need many more. I hope that Rome will send out directives to bishops around the world calling on them to appoint more exorcists."

He said John Paul II had carried out many exorcisms during his pontificate, but that Benedict XVI had not performed any, instead leaving it to bishops and priests.

"John Paul II fought many times against Satan," Father Amorth said. "Those battles continue, even though he is dead. In fact he is present today in many exorcisms. If you cite his name during an exorcism, the person who is possessed literally froths at the mouth in fury."

Francis has made repeated references to the Devil since he was chosen as Benedict's successor during a secret conclave in March.

At his first Mass, which was held in the Sistine Chapel, where he had been elected by his brother cardinals, he warned: "If we do not confess, if we do not pray to the Lord, we are praying to the Devil. When you do not confess to Jesus Christ, you are confessing to the worldliness of the Devil."

People possessed by evil are said to display certain symptoms, from extraordinary physical strength to speaking "in tongues" and vomiting nails and bits of glass.

Catholic priests who have trained as exorcists have to follow a ritual known as "De exorcismis et supplicationibus quibusdam", or "Of exorcisms and certain supplications", according to guidelines laid down by the Church.

The 84-page document was drawn up in 1998 and is an update on an exorcism rite which was compiled in 1614.