Pope Francis thrilled tens of thousands of people gathered for his installation Mass today, taking a long driveabout through St Peter's Square and getting out of his vehicle to bless a disabled man in a wheelchair in the crowd.

Blue and white flags from his native Argentina fluttered above the Vatican City crowd which Italian media estimate could reach a million.

Civil protection crews closed the main streets leading to the square to traffic and set up barricades for nearly a mile along the route to try to control the masses.

For nearly half an hour, Francis toured the square in an open-topped car, waving and occasionally kissing babies handed up to him as if he had been doing it for years.

At one point, as he neared a group of people in wheelchairs, he signalled for the vehicle to stop, got off and went to bless a man held up to the barricade by an aide.

Pope Francis urged princes, world leaders and thousands of ordinary people at his installation Mass to protect the environment, the weakest and the poorest.

The Mass was a simpler affair than the 2005 ceremony which launched Pope Benedict XVI's papacy, in keeping with Francis's sober style, but was still grand enough to draw 132 official delegations and religious leaders from around the world.

Among the VIPs were the spiritual leader of the world's Orthodox Christians, Bartholomew I, who became the first patriarch from the Istanbul-based church to attend a papal investiture since the two branches of Christianity split nearly 1,000 years ago. His presence underscored the broad hopes for ecumenical and interfaith dialogue in this new papacy.

But it is Francis's history of living with the poor and working for them while Archbishop of Buenos Aires which seems to have resonated with ordinary Catholics who say they are hopeful that he can inspire a new generation of faithful who have fallen away from the Church.

"I think he'll revive the sentiments of Catholics who received the sacraments but don't go to Mass anymore, and awaken the sentiments of people who don't believe anymore in the Church, for good reason," said Judith Teloni, an Argentine tourist guide who lives in Rome and was attending the Mass with a friend.

Francis has made headlines with his simple style since the moment he appeared to the world on the balcony of St Peter's Basilica, eschewing the ermine-lined red velvet cape his predecessor wore in favour of the simple papal white cassock, then paying his own bill at the hotel where he stayed prior to the conclave that elected him pope.

During today's Mass, Francis received the woollen pallium, or stole, symbolising his role as shepherd of his flock, and also the simple gold-plated silver fisherman's ring which is a symbol of the papacy.

A wax cast of the ring was first presented to Pope Paul VI, who presided over the second half of the Second Vatican Council, the 1962-65 meetings that revolutionised the church. Paul never wore it but the cast was subsequently made into the ring that Francis chose among several other more ornate ones.

Francis will receive each of the government delegations in St Peter's Basilica after the Mass, and then hold an audience with the visiting Christian delegations tomorrow.

He will have a break on Thursday; a gracious nod perhaps to the fact that the Archbishop of Canterbury, the Most Rev Justin Welby, is being installed that day in London.

As a result, Mr Welby will not be representing the Anglican Communion at today's installation Mass for Francis, sending instead a lower-level delegation. All told, six sovereign rulers, 31 heads of state, three princes and 11 heads of government will be attending, the Vatican said.

More than half a dozen Latin American presidents are attending, a sign of the significance of the election for the region. Francis, named after the 13th century friar known for his care of the most disadvantaged, has made clear he wants his pontificate to be focused on the poor, a message that has resonance in a poverty-stricken region that counts 40% of the world's Catholics.

For Jews, Orthodox and other religious leaders, the new pope's choice of Francis as his name is also important for its reference to the Italian town of Assisi, where Pope John Paul II began conferences encouraging interfaith dialogue and closer bonds among Christians.

Francis was interrupted by applause several times during his homily, including when he spoke of the need to protect the environment, serve one another with love and not allow "omens of destruction," hatred, envy and pride to "defile our lives."

Francis said the role of the pope is to open his arms and protect all of humanity, but "especially the poorest, the weakest, the least important, those whom Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick and those in prison."

"Today amid so much darkness we need to see the light of hope and to be men and women who bring hope to others," he said. "To protect creation, to protect every man and every woman, to look upon them with tenderness and love, is to open up a horizon of hope, it is to let a shaft of light break through the heavy clouds," he said.

Francis, 76, thrilled the crowd at the start of the Mass by taking a trip through the piazza and getting out of his jeep to bless a disabled man. It was a gesture from a man whose short papacy so far is becoming defined by such spontaneous forays into the crowd and concern for the disadvantaged.

Before the Mass began, Francis received the fisherman's ring symbolising the papacy and a wool stole symbolising his role as shepherd of his flock.

A cardinal intoned the rite of inauguration, saying: "The Good Shepherd charged Peter to feed his lambs and his sheep; today you succeed him as the bishop of this church."

Some 132 official delegations attended, including more than a half-dozen heads of state from Latin America, a sign of the significance of the election for the region. Francis has made clear he wants his pontificate to be focused on the poor, a message that has resonance in a poverty-stricken region that counts 40% of the world's Catholics.

In the VIP section was German chancellor Angela Merkel, US vice president Joe Biden, the Argentine president Cristina Fernandez, Taiwanese president Ying-Jeou Ma, Zimbabwe president Robert Mugabe, prince Albert of Monaco and Bahrain prince Sheik Abdullah bin Haman bin Isa Alkhalifa, among others. Six sovereign rulers, 31 heads of state, three princes and 11 heads of government attended.

Francis directed his homily to them, saying: "I would like to ask all those who have positions of responsibility in economic, political and social life, and all men and women of goodwill: let us be protectors of creation, protectors of God's plan inscribed in nature, protectors of one another and of the environment."

Among the religious VIPs attending was the spiritual leader of the world's Orthodox Christians, Bartholomew I, who became the first patriarch from the Istanbul-based church to attend a papal investiture since the two branches of Christianity split nearly 1,000 years ago. Also attending for the first time was the chief rabbi of Rome. Their presence underscores the broad hopes for ecumenical and interfaith dialogue in this new papacy given Francis' own work for improved relations and St. Francis of Assisi.

Francis has surprised - and perhaps frustrated - his security detail by his impromptu forays into the crowds.

In an indication of his devotion to the Virgin Mary, which is common among Latin American Catholics, Francis prayed by a statue of the Madonna at the end of the service.

After the Mass, Francis stood in a receiving line to greet each of the government delegations, chatting with each one, kissing the few youngsters who came along with their parents and occasionally blessing a rosary given to him.