The 266th pontiff has officially begun his ministry as leader of the world's 1.2bn Catholics after a ceremony which began with a hint of informality and fun.
Speaking at an outdoor Mass to mark his inauguration, he called on world leaders to respect the environment and protect the weak.
Pope Francis also personally sent his best wishes to the people of Ireland and asked them for their prayers.
In his sermon to 200,000 gathered in St Peter's Square, the Pope struck a serious note by speaking of the importance of protecting the "beauty of the created world" and humanity, especially children, the poor and those in need – calling on those in "positions of responsibility" to protect creation.
"Whenever human beings fail to live up to this responsibility, whenever we fail to care for creation and for our brothers and sisters, the way is opened to destruction and hearts are hardened," he said. "Tragically, in every period of history there are 'Herods' who plot death, wreak havoc, and mar the countenance of men and women.
"Please, 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 in nature."
The event was attended by VIP guests from all over the world .
The Queen was represented by the Duke of Gloucester. Prince Albert and his wife Charlene represented Monaco.
After his formal inauguration, the Pope met with Irish President Michael D Higgins.
During the brief encounter inside St Peter's Basilica, the pontiff asked Mr Higgins to pass on his greetings to the Irish people.
"Best wishes to the people of Ireland," he said.
"And I ask them for their prayers."
Earlier, a relaxed Francis thrilled the crowd by getting out of his vehicle to bless a disabled man.
He shouted "ciao!" to wellwishers and stopped to kiss babies.
In a break with tradition, he walked through the crowds, a signal of his more relaxed style.
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