House Democratic Leader Nancy Pelosi accompanied Biden in the U.S. delegation, the White House said. On Friday, House Speaker John Boehner, a Republican, said he would send a separate bipartisan congressional delegation.

Cardinal Timothy Dolan of New York, in a letter sent to Francis on Tuesday, offered the "deep affection, prayerful support and sincere pledge of fidelity" of the U.S. bishops and more than 70 million Catholics in the country.

Also at the Vatican were groups from the Americas, including Brazil, Chile, Mexico and Canada, and from European nations such as Italy, Holland, Belgium and Germany.

Representatives from across Christianity -- Eastern and Western -- were also present, along with members of other religions, including Judaism, Islam, Buddhism, Sikhism and Jainism.

This is also one of the busiest times of the year on the Christian calendar, which will mean many public appearances for the new pontiff.

Less than a week away is Palm Sunday, the holiday that kicks off Holy Week, which culminates in Easter celebrations.

The Vatican press office said Francis will also have one other task to keep him busy: figuring out "how to answer the millions of e-mail messages that have already been received, despite the fact that he still doesn't have an official address."