The pontiff released a special encyclical lamenting the exploitation of the Earth's environment and resources, often at the expense of the poor.
Francis went to Villa Borghese, a popular park in the Italian capital.
#PopeFrancis made surprise visit to #EarthDay event in Villa Borghese - benvenuto Papa! pic.twitter.com/mHChs09Nyf
— Elise Harris (@eharris_it) April 24, 2016
The pope sat on an outdoor stage along with an Italian priest who denounced Naples-area mobsters for systemically dumping toxic waste on local farmland in a lucrative, illicit operation often serving legitimate businesses.
The pope told the audience that money has become a god "at the centre of the world".
He said: "There are so many deserts in the cities, in the lives of people who have no future, because there are...always prejudices and fears.
"We must not be afraid to go to the desert and transform it into a forest, where there's abundant life."
