VATICAN CITY — In his first public appearance since the announcement of his resignation, Pope Benedict XVI said Wednesday that he had made his decision “in full liberty for the good of the church” because he no longer had the strength needed to carry out the duties of the papacy.

Wearing simple white robes and a skullcap at a general audience in the Vatican, the pope spoke as Christians began Lent, the 40-day period of fasting and prayer preceding Easter. Later he celebrated the Ash Wednesday Mass at St. Peter’s Basilica, an event that his aides said would probably be his final major Mass in the huge edifice before his retirement.

The basilica was packed for the occasion, and the pope presided in the purple robes that priests traditionally wear during Lent. The Ash Wednesday Mass has usually been celebrated by the pope in a midsize church on the Aventine Hill in Rome, but the venue was changed after the pope announced his resignation, to allow more people to attend the service.

The pope walked into the basilica unaided, but then stood on a wheeled platform that transported him among the many congregants in the immense basilica.

The announcement Monday of the first papal resignation in almost 600 years touched off a behind-the-scenes contest among the cardinals of the Roman Catholic Church to succeed him.

The conclave to choose the next pope will begin in mid-March, 15 to 20 days after Benedict steps down, and a new leader of the church is expected to be in place by Easter, which falls on March 31 this year.

The Rev. Federico Lombardi, the Vatican spokesman,said the cardinals would begin to meet before the conclave in plenary sessions for discussions so that they go into the conclave “informed.”

As that drama plays out, Benedict will spend his final day as pope bidding farewell to cardinals in the morning. Then he plans to fly by helicopter in the afternoon to the papal summer residence at Castel Gandolfo, in the hills outside Rome, where he will be when his resignation takes effect at 8 p.m. local time, Lombardi said.

His meeting with cardinals will be informal, “not solemn” and “very personal,” according to Lombardi, who has said Benedict will not seek to influence their decision about a successor, even though many of them are kindred conservatives, appointed either by him or by John Paul II.

Lombardi said at a news conference that the pope would hold scheduled audiences with the Romanian and Guatemalan presidents this week and with the Italian president Feb. 23.

But there will be no formal ceremony of resignation, he said, because Benedict has already fulfilled the canonical requirements for his departure by affirming publicly and in Latin - as he did to an assembly of cardinals Monday - that he was resigning of his own free will.

Lombardi said Benedict decided to repeat his decision Wednesday before an “assembly of people of God” at his scheduled weekly audience, but the spokesman declined to respond directly to the question of whether the pope would receive a pension. Lombardi said the church would “obviously ensure that he would be looked after satisfactorily,” but that “he’s not going to be a burden on the expenses of the Holy See.”

Earlier, a cheering crowd greeted the pope with a standing ovation as he entered the Vatican’s Paul VI audience hall, which has a capacity of around 8,000. A huge banner at the rear proclaimed: “Thanks Your Holiness.” He smiled often and thanked anItalian children’s choir that sang for him in German, his native tongue, saying, “The gift of singing songs is particularly dear to me.”

Before reaching his decision to resign, the pope said, he prayed and examined hisconscience for a long time. Referring to the papacy, he said he had been “well aware of the seriousness of this act, but also aware of the fact that I am no longer capable of carrying out Peter’s Ministry with the strength needed.”

“The certainty that the church belongs to God supports and illuminates me, and Christ will always give his guidance and care,” he told the audience. “I thank you all for your love and prayer with which you’ve accompanied me. Please keep praying for the pope and the church. I felt it almost physically, throughout these days that were not easy for me. Keep praying for me, for the church and for the future pope. The Lord will guide us.”

The pope’s appearances Wednesday offered his followers a chance to see and hear him before he withdraws into a far more sheltered life in a former convent withinthe Vatican walls, where an apartment has been prepared for him.

Information for this article was contributed by Gaia Pianigiani of The New York Times.

Front Section, Pages 5 on 02/14/2013