With Palm Sunday less than a week away, the inauguration comes during one of the busiest times of the year on the Christian calendar. And the day was part solemn ritual, part public celebration.
Before mass, the pope thrilled the crowd by taking about a half-hour ride through the square in an open-air jeep. Along the way, he stopped and reached out to some of the estimated 150,000 to 200,000 people who gathered.
Still wearing his simple iron cross he wore as a cardinal, the pope accepted the gold fisherman's ring and wool stole, symbols of the papacy.
In his homily, the pope urged protection of the environment, the weakest and the poorest, mapping out a clear focus of his priorities as leader of the world's 1.2 billion Catholics.
Representatives from 132 countries and international organizations were expected to attend the installation Mass. Vice President Joe Biden met with Italy's Prime Minister Mario Monti in Rome. Biden is the first Roman Catholic to serve as vice president, and he led the U.S. delegation for the formal installation of the pope.
The Vatican said no one was officially invited to the inaugural Mass, but everyone was welcome to come. And that included the pope's former adversary, the president of his native Argentina, Christina Kirchner. The two met Monday and interacted like they were old friends. But when Francis was a cardinal in Argentina, Kirchner described his social views as "medieval."
He often clashed with Kirchner over legalizing gay marriage, calling it the "devil's handiwork." He called gay adoption "discrimination against children" and he's denounced abortion as promoting "a culture of death." But the powerful Catholic strongholds of Argentina and Brazil are becoming more progressive. Gay marriage is now legal and contraception is readily available.
So far, the pope's conservative stance on social issues appears to be having no impact on his popularity. Francis is quickly becoming known as the people's pope, mingling with parishioners like a local priest.
While Francis has already made an impression as a pope of the people concerned about the welfare of the poor, the test is yet to come to see what kind of difference he can make inheriting a church wracked by a decades-old sexual abuse scandal and claims of corruption in the clergy.
The Associated Press contributed to this report.