Huge crowds in the Vatican cheered Pope Benedict XVI on Wednesday as he made his first public appearance since announcing his resignation at the end of the month.
He thanked the Roman Catholic faithful in several languages and said it was not appropriate for him to continue as pope.
"I've decided to resign the ministry given to me by the Lord. I've done this in full freedom, for the benefit of the church," he told those gathered in a Vatican City audience hall for his regular weekly appearance.
Benedict said he is fully aware of the gravity of his decision but he could not carry on as pope "if I don't have the strength that it requires."
He appeared tired but not visibly unwell as he sat and read his remarks off several sheets of paper.
Benedict later celebrated an Ash Wednesday mass that marked the beginning of Lent at St. Peter's Basilica in the afternoon.
The service was moved from St. Sabina Church, where it is traditionally held, to the Basilica to accommodate the larger crowd of worshipers, Vatican officials said. News Monday that Benedict was stepping down at the end of February "because of advanced age" shocked the world's 1.2 billion Roman Catholics.
He is the first pope to resign in nearly 600 years.
Benedict will keep working until February 28, meeting with pastors from Rome, Italian bishops, and the visiting presidents of Romania, Guatemala and other countries, according to the Vatican press office.
He will hold a final audience in Vatican City's St. Peter's Square on February 27, but the church is not planning a formal ceremony to mark the pope's departure, the Rev. Federico Lombardi, a spokesman for the Vatican, said Wednesday.
Benedict will simply leave at the end of his typical workday -- 8 p.m. in Vatican City, Lombardi said.
The pontiff, born Joseph Ratzinger, will first go to the pope's summer residence in Castel Gandolfo, and then is likely to retire to a monastery and devote himself to a life of reflection and prayer, Lombardi said.
He won't be involved in managing the church after his resignation.
"The pope is well and his soul is serene. He did not resign the pontificate because he is ill but because of the fragility that comes with old age," Lombardi said.
Vatican Secretary of State Tarcisio Bertone on Wednesday thanked Pope Benedict for his service.
"All of us have understood that it is precisely the deep love that your holiness has for God and the church which pushed you to this act," he said.
The pope's decision was not linked to a recent medical intervention to replace the battery in his pacemaker, Lombardi said, adding that it had been a routine procedure.
Benedict's unexpected move has inevitably prompted frenzied speculation over who might assume the papacy in his place.
The Vatican does not yet know when the cardinals will meet in a conclave to decide who will replace Benedict, but it must begin within 20 days of the pope's resignation, Lombardi said.
He had previously said a new pope would be in place before Easter is celebrated at the end of March.