VATICAN CITY (AP) -- Pope Benedict XVI is celebrating his last public Mass as pontiff, presiding over Ash Wednesday services inside the packed St. Peter's Basilica in Rome.
Looking tired, the 85-year-old Benedict was wheeled down the central aisle as a choir sang, preceded by dozens of cardinals.
Ash Wednesday marks the start of Lent, the most solemn season in the church's liturgical calendar that ends with Holy Week and Easter Sunday.
Wearing heavy purple vestments, Benedict referred to his pending Feb. 28 resignation in his homily, asking for the prayers of the faithful.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
Pope Benedict XVI told thousands of faithful Wednesday that he was resigning for "the good of the church" - an extraordinary scene of a pope explaining himself to his flock that unfolded in his first appearance since dropping the bombshell announcement.
Looking tired but serene, the 85-year-old Benedict basked in a standing ovation when he entered the packed hall for his traditional Wednesday catechism lesson. His speech was interrupted repeatedly by applause, and many in the audience of thousands had tears in their eyes.
A huge banner reading "Grazie Santita" (Thank you Your Holiness) was strung up at the back of the room.
Benedict appeared wan and spoke very softly, but his eyes twinkled at the flock's warm and heartfelt welcome. He repeated in Italian what he had told his cardinals Monday in Latin: that he simply didn't have the strength to continue.
"As you know, I have decided to renounce the ministry that the Lord gave to me on April 19, 2005," he said, to applause. "I did this in full liberty for the good of the church."
He thanked the faithful for their prayers and love, which he said he had "physically felt in these days that haven't been easy for me." And he asked them to "to continue to pray for me, the church, and the future pope."
The atmosphere was festive and warm, if somewhat bittersweet, as if the crowd was trying to ask him to stay with them for just a bit longer. A chorus of Italian schoolchildren serenaded him with one of his favorite hymns in German - a gesture that won over the pope, who thanked them for singing a piece "particularly dear to me."
"He gave us eight wonderful years of his words," said Ileana Sviben, an Italian from the northern city of Trieste who couldn't hide her sadness. "He was a wonderful theologian and pastor."
"We were just coming for vacation, and now we are getting all of this!" marveled Terry Rodger, a tourist from New Orleans as he headed to the audience. "I am very excited. I'm surprised."
But the Rev. Reinaldo Braga Jr., a Brazilian priest studying theology in Rome, said he was saddened when he first heard the news.
"The atmosphere was funereal but nobody had died," he said. "But then I realized it was a wise act for the entire church. He taught the church and the world that the papacy is not about power but about service."
It was a sentiment the retiring Benedict himself emphasized Wednesday when he told his flock that the "path of power is not the road of God."
The audience included groups of nuns waving papal flags, and among the clerics, U.S. Cardinal Bernard Law, who resigned as archbishop of Boston at the height of the clerical sex abuse scandal in the United States.
Benedict is the first pope to resign in nearly 600 years, and the decision has placed the Vatican in uncharted waters: No one knows what he'll be called or even what he'll wear after Feb. 28.
The Vatican, however, revealed some details of his final day as pope, saying he would attend a morning farewell ceremony with his cardinals and then fly off by helicopter at 5 p.m. to the papal summer retreat at Castel Gandolfo.
Under that timetable, Benedict will be far from the Vatican when he ceases being pope at 8 p.m. - a deadline decided by Benedict himself because that's when his normal workday ends.
Vatican spokesman the Rev. Federico Lombardi said no formal or symbolic act was needed to make his resignation official at that time, because Benedict has already done all that was required to resign by affirming he had taken the decision freely.
Benedict's final official acts as pope will include audiences with the Romanian and Guatemalan presidents this week and the Italian president on Feb. 23.
Making sure the transition goes smoothly, Benedict made an important appointment Wednesday, naming the No. 2 administrator of the Vatican city state, Monsignor Giuseppe Sciacca, as a legal adviser to the camerlengo.
The camerlengo, or chamberlain, helps administer the Vatican bureaucracy in the period between Benedict's resignation and the election of a new pope.
The current camerlengo is Benedict's longtime trusted aide, Cardinal Tarcisio Bertone, the Vatican secretary of state. Bertone takes over the moment Benedict retires and will play a key role in organizing and participating in the conclave to elect a new pope, expected sometime in mid-March.
At that conclave, the church's 117 cardinals under the age of 80 will get to vote on who should succeed Benedict.
The Vatican has made clear that Benedict will play no role in the election of his successor, and once retired, he will be fully retired. He plans to live a life of prayer in a converted monastery on the far northern edge of the Vatican gardens.
But his continued physical presence within the Vatican walls has raised questions about how removed he really will be from the life of the church. Lombardi acknowledged that Benedict would still be able to see his friends and colleagues.
"I think the successor and also the cardinals will be very happy to have very nearby a person that best of all can understand what the spiritual needs of the church are," Lombardi said.
Benedict is expected however, to keep a low public profile.
As a result, Benedict's final public appearances - his last general audience will be Feb. 27 - are expected to draw great crowds, as they may well represent some of the last public speeches for a man who has spent his life - as a priest, a cardinal and a pope - teaching and preaching.
And they will also give the faithful a way to say farewell under happier circumstances than when his predecessor, Pope John Paul II, died in 2005.
The audience was the start of a busy day for Benedict: he will also preside over Ash Wednesday services later in the day to mark the official start of the Catholic Church's solemn Lenten season. The service is usually held in a church on Rome's Aventine hill, but was moved at the last minute to St. Peter's Basilica. The Vatican said the shift was made to accommodate the crowds, though it will also spare the pope the usual procession to the church.
The Vatican insisted no serious medical ailment was behind Benedict's decision to retire, though it admitted for the first time on Tuesday that Benedict has had a pacemaker for years and recently had its battery replaced.