Addressing up to 200,000 people and many foreign leaders gathered under bright sunshine in St. Peter's Square on Tuesday, the new Argentine Pope Francis underlined his constant message since he was elected by a secret conclave of cardinals last Wednesday - which the Church's mission was to defend the poor and disadvantaged.
In line with this message, the inaugural Mass on the steps of the giant St. Peter's Basilica was much simpler and an hour shorter than the baroque splendour of his predecessor Benedict's inauguration in 2005.
The Mass formally installed Francis as head of the world's 1.2 billion Roman Catholics.
The Church's mission "means respecting each of God's creatures and respecting the environment in which 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.
The message chimed with the teachings of the 13th century St. Francis of Assisi, from whom the pope took his name and who is a symbol of poverty, simplicity, charity and love of nature.
Francis 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 crisis.
"For me this is a call to humility and service to others that will mark his papacy... This is a new breeze of fresh air that is blowing through the Church and the name of that breeze is Francis," Argentinian Cardinal Leonardo Sandri told Reuters.
That reaction was shared by many.
"He is a simple, humble person, he is not like the untouchable popes, he seems like someone normal people can reach out to," said Argentine electrician Cirigliano Valetin, 51, who works in southern Italy and was among the huge crowd.
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."
Before the Mass, the pope toured St. Peter's Square, which was filled with people waving flags, in an open white jeep, abandoning the bullet-proof popemobile often used by Benedict.
He stopped frequently to greet those in the sprawling square, kissing babies and getting out to bless a disabled man.
For the Mass he wore plain white vestments, trimmed with gold and brown, and black shoes, in contrast to the red ones that Benedict wore and that attracted attention.
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.
"We have gone from the rigid theology professor to a teacher of Christian simplicity," said Italian church historian Alberto Melloni, referring to the contrast between the warm, common touch of Francis and Benedict's stiff, intellectual manner.
Although he is as conservative doctrinely as Benedict, his simplicity and his emphasis on the Church's mission to care for the poor is a sea change from his predecessor.
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 a 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, that had been 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.
Many in the crowd said they had high hopes of a more humble papacy under Francis, who as a Jesuit has taken a vow of poverty and who said 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, Argentine President Cristina Fernandez, 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.
Francis greeted the foreign delegations inside the basilica after the Mass. They included 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. He was able to travel to the Vatican because it is a separate territory, outside the EU.
The former Buenos Aires Cardinal Jorge 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.
Before the Mass began, Latin America's first pope made a surprise phone call to thousands of his Catholic compatriots, who gathered for hours before dawn in front of large television screens in the central Plaza de Mayo square in Buenos Aires.
In his telephone message at 3.30 a.m. local time (0630 GMT), Francis thanked the crowd for their prayers "which I need a lot".
"I want to ask you a favour, that we we walk together, that we look after each other... Don't forget this bishop, who though far away, cares so much for you," he said.