The stunned crowd could only balk as the man was quickly torn away from the altar by "plainclothes police officers" stationed at the basilica and escorted off of the grounds.
The pope invoked his predecessor, Francis, for whom the environment was a core issue, but stopped short of criticizing world leaders dismissive of climate change.
In his first major appointment, Leo appointed Archbishop Filippo Iannone, 67, the new prefect for the Dicastery of Bishops, which advises the pope on which priests should become bishops.