VATICAN CITY: Starting his public farewell to his flock, a weary Pope Benedict XVI celebrated his final public Mass as pontiff, presiding over Ash Wednesday services hours after a bittersweet audience that produced the extraordinary scene of the pope explaining his decision to step down directly to the faithful.

The mood inside St. Peter’s Basilica was somber during the Mass, as if the weight of Benedict’s decision and the finality of his pontificate had finally registered with the thousands of faithful present. But the basilica erupted in a rousing, minuteslong standing ovation as Benedict exited for the last time as pope, bringing tears to the eyes of some of his closest collaborators.

“We wouldn’t be sincere, Your Holiness, if we didn’t tell you that there’s a veil of sadness on our hearts this evening,” Cardinal Tarcisio Bertone, Benedict’s longtime deputy, said at the end of the service, his voice breaking.

“Thank you for having given us the luminous example of the simple and humble worker in the vineyard of the Lord,” he added, quoting Benedict’s own words when he first appeared on the loggia overlooking St. Peter’s Square after he was elected pope.

“Viva il papa!” the crowd yelled as Benedict stepped off the altar.

Ash Wednesday marks the start of Lent, the most solemn season on the church’s liturgical calendar that ends with Holy Week, when the faithful commemorate the death of Christ and his resurrection on Easter Sunday. By this Easter, on March 31, the church will likely have a new pope.

The scene was festive earlier in the day, when Benedict took the extraordinary step of speaking directly to his flock about why he had broken with 600 years of tradition and decided to retire on Feb. 28.

“As you know, I have decided to renounce the ministry that the Lord gave to me on April 19, 2005,” Benedict said, to warm 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.”

Benedict looked tired but serene as he 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 and a chorus of Italian schoolchildren serenaded the pope with one of his favorite hymns in German – a gesture that won over the pontiff, who thanked them for singing a piece “particularly dear to me.”

He appeared wan and spoke very softly, but his eyes twinkled at the flock’s welcome – warm and heartfelt if somewhat bittersweet.

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.”

Benedict’s decision to resign 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 he himself decided because that’s when his normal workday ends.

Vatican spokesman Rev. Federico Lombardi said no formal or symbolic act was needed to make his resignation official at that time, because Benedict had already done all that was required to resign by affirming publicly that 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 Italy’s 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 a new pope’s election.