In his first tweet after his inauguration on the @Pontifex account, Pope Francis said: "Let us keep a place for Christ in our lives, let us care for one another and let us be loving custodians of creation."
He then tweeted again: "True power is service. The Pope must serve all people, especially the poor, the weak, the vulnerable."
Argentines watch Mass
Back in Francis' home city, the Argentinian capital, Buenos Aires, the faithful packed the main square to watch the event on large screens set up throughout the Plaza de Mayo.
According to the state-run newspaper Telam, a telephone call Francis had made from the Vatican was played to all present, saying: "Thanks for the prayers."
He added, "Let us be aware of one another, care for life, nature, children and old people," before concluding by asking those present to pray for him.
Groups also gathered in other cities throughout Argentina to watch the early morning Mass, Telam reported.
The event in St. Peter's Square was attended by 132 delegations from around the world, including six reigning sovereigns and 31 heads of state.
Those delegations are among scores from nations and international organizations traveling to the Vatican, led by heads of states and governments.
European Union leaders Jose Manuel Barroso and Herman Van Rompuy were among those present.
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."