Francis said he willingly joined their protest "in favor of family and life, which in these times require special pastoral and cultural attention around the world."
Plenty of subtexts will play out over 90 minutes of must-see TV before an estimated audience of 75 million or more viewers. Here's what you should look out for.
Many saw the massive march as the Roman Catholic church flexing its political muscle in a country where about 80 percent identify as nominally Catholic.