Pope Francis has made a surprise visit to one of Rome's main parks to participate in an Earth Day event.

Francis, who wrote an encyclical decrying the exploitation of the Earth's environment and resources often at the expense of the poor, went to Villa Borghese, a park filled with Sunday strollers.

He sat on an outdoor stage along with an Italian priest who has courageously 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 center of the world."