Addressing an estimated 200 000 people and many foreign leaders gathered under bright sunshine in St Peter's Square on Tuesday, the Argentinian pope underlined his constant message since he was elected by a secret conclave of cardinals last Wednesday – that the church's mission was to defend the poor and disadvantaged.
In line with this message, the Mass on the steps of the giant St Peter's Basilica was simpler than the baroque splendour of his predecessor Benedict's inauguration in 2005.
The church's mission "means respecting each of God's creatures and respecting the environment in that we live. It means protecting people, showing loving concern for each and every person, especially children, the elderly, those in need, who are often the last we think about", he said in the homily.
Francis, the former Cardinal Jorge Bergoglio of Argentina, took his name in honour of St Francis of Assisi, a symbol of poverty, simplicity, charity and love of nature.
He said that whenever human beings failed to care for the environment and each other, "The way is opened to destruction and hearts are hardened. Tragically in every period of history there are 'Herods' who plot death, wreak havoc and mar the countenance of men and women."
The hallmark simplicity of Francis, the first Jesuit pope, has fuelled hopes for change and renewal in a church beset by a deep global crisis.
"He is a simple, humble person, he is not like the untouchable popes, he seems like someone normal people can reach out to," said one of those in the huge crowd, Argentinian electrician Cirigliano Valetin (51) who works in southern Italy.
Francis inherits a church mired in scandals over priests' sexual abuse of children and the leak of confidential documents alleging corruption and rivalry between cardinals inside the church government or Curia.
He has also been accused by some critics in Argentina of not doing enough to oppose human rights abuses under a military government during the 1976-1983 "dirty war" when some 30 000 leftists were kidnapped and killed. The Vatican has strongly denied the accusations.
In his homily, the new pope called for world leaders to be "protectors of one another and of the environment … Let us not forget that hatred, envy and pride defile our lives. Being protectors, then, also means keeping watch over our emotions, over our hearts."
Pope toured square in open Jeep
Before the Mass, the pope toured St Peter's Square, which was crammed with people waving flags, in an open white jeep, abandoning the bulletproof popemobile often used by Benedict.
He stopped frequently to greet those in the sprawling square, kissing babies and getting out at one point to bless a disabled person.
For the Mass he wore plain white vestments, trimmed with gold and brown, and black shoes, in contrast to the luxurious red loafers that attracted attention under Benedict.
The ceremony, conducted from an altar on the steps of the huge basilica, was shortened to two hours after a three-hour service in 2005 when Benedict began his papacy.
After the Mass hundreds of priests, sheltering from the sun under umbrellas in the Vatican's white and yellow colours, distributed communion to the crowd while Francis watched from his raised throne behind the altar.
Before the Mass, Francis collected his newly minted gold ring and pallium, a liturgical woollen band worn around the neck, which was placed overnight on the tomb of St Peter under the basilica's altar.
He processed out of the church in a column of cardinals chanting a litany calling for support for the new pontiff from saints, including several previous popes.
The Mass formally installs Francis as the new leader of the world's 1.2-billion Roman Catholics.
Many in the crowd said they have high hopes of a more humble papacy under Francis, who as a Jesuit has taken a vow of poverty and who said he would be inspired by the lowly.
"My first impression is that the pope is very humble, and has taken the church in his heart," said Isaac Adroamabe from Arua in Uganda, who is studying to be a priest in Rome.
"I think he is going to fulfill his promises, he will lead the church based on the example of St Francis, you can already see he is a down-to-earth pope who mingles with the people."
Six sovereigns, US Vice-President Joe Biden, Argentina's President Cristina Fernández, other leaders as well as heads of many other faiths were among the 130 delegations on the steps of the famous basilica.
Among them was Ecumenical Patriarch Bartholomew from Istanbul, the first time the spiritual head of Orthodox Christians has attended a Roman pope's inaugural Mass since the Great Schism between western and eastern Christianity in 1054.
Bergoglio has aroused enormous enthusiasm and interest in the Catholic world due to the modest way he has assumed a post that was modelled after a Renaissance monarchy and carries titles such as "Vicar of Jesus Christ" and "Supreme Pontiff of the Universal Church".
In the six days since his election, he has referred to himself only as Bishop of Rome, the position from which his authority flows, and hinted he plans to reduce Vatican centralism and govern in consultation with other bishops.
Francis will receive the visiting political leaders in the basilica after the Mass.
Francis had his first taste of the diplomatic challenges of the papacy when on Monday, Fernández asked him to support Buenos Aires in a dispute with Britain over the Falkland Islands in the South Atlantic.
A Vatican spokesperson had no comment on the request.
Mugabe's travel ban
He will also find himself greeting an international pariah, Zimbabwe President Robert Mugabe, who has been under a European Union travel ban since 2002 because of allegations of vote rigging and human rights abuses.
The Vatican is not part of the European Union, allowing Mugabe to travel there.
The Vatican on Monday revealed the new pope's coat of arms, similar to the one he used as Archbishop of Buenos Aires, with symbols representing Jesus, Mary and Joseph.
Symbols of the papacy have been added behind it, including two keys that signify the Biblical passage in which Jesus told St Peter he would give him "the keys of the kingdom" of heaven.
The motto on his coat of arms is "miserando atque eligendo" (having had mercy, he called him), which comes from a meditation by the Venerable Bede, an 8th century English monk, on a passage of the Gospel in which Jesus calls St Matthew to be an apostle.
In various sermons and comments since his election last Wednesday, the pope has urged people to be more merciful and not to be so quick to condemn the failings of others. – Reuters