Style or substance?

The Mass which inaugurated Francis as bishop of Rome and marked the official start of his papacy was short, lasting about two hours. This was in keeping with the spirit of simplicity embraced by the new pontiff, the Vatican said before the occasion.

Francis has already made an impression as a pope of the people who is concerned about the welfare of the poor. But he inherits a church wracked by a decades-old sexual abuse scandal and claims of corruption in the clergy.

Monsignor Kevin Irwin told CNN the Catholic Church probably would not have to wait long before the changes in style already seen under Francis become changes in substance.

"It comes down to personnel," he said. "The personnel changes you make in any corporation make all the difference. Leadership matters -- but then who are the other people who help implement his program?"

The Vatican said Saturday that Francis had provisionally confirmed members of the Vatican hierarchy in their positions, but would take time to reflect before any final appointments are confirmed.

Irwin said he believes the key changes are likely to come in the late spring or early summer.

Francis, as a member of the Jesuit order, "gets into things" and is tenacious, he said. "This is a 'take-charge' pope, I would say."

World figures

Back in St. Peter's Basilica, the newly installed bishop of Rome greeted the dignitaries who had flocked from around the world to attend the inauguration Mass.

President Cristina Fernandez de Kirchner, from Francis' native Argentina, was the first head of state to step up.

Vice President Joe Biden, leading the U.S. presidential delegation, was also among those to meet the 76-year-old pope, as was Zimbabwe's controversial President Robert Mugabe.

Mugabe is subject to a European Union travel ban but allowed to visit the continent for religious events and international conferences.

Vatican Secretary of State Cardinal Tarcisio Bertone -- effectively the Vatican's prime minister -- was at the pope's side as, standing, he gave his greetings for close to an hour and a half.

The formal encounters over, Francis then went to the barriers around the sides of the basilica to greet priests, nuns and other well-wishers.

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.