"Lent is, in the end, through almsgiving, the occasion for sincere sharing of gifts received with one's brothers and for attention to the needs of the poorest and abandoned ones," Benedict told 10,000 pilgrims and tourists at his weekly general audience in St. Peter's Square.
In early evening, Benedict led a procession outside a basilica on the Aventine Hill in a ceremony that included the placing of ashes on the heads of the faithful, a gesture symbolizing mortality.
Dressed in purple vestments, the pope placed ashes on the heads of cardinals, bishops and rank-and-file Catholics, all kneeling before him.
In brief remarks, he said the Christian response to threats against peace in the world must not be "vendetta, hate or escape to false spiritualism" but to follow Christ's teachings.
Ash Wednesday launched a busy season for the pope. It will be capped by Holy Week, the seven days starting with Palm Sunday a week before Easter on April 16.
Benedict told the audience at the Vatican that the imposition of ashes, a ritual conducted in all Catholic churches, is an "austere and symbolic gesture."
He urged the faithful to "look with fresh eyes at their brothers and their needs."
Benedict denounced "indifference and enclosure in one's own selfishness" in the face of the "terrible challenges of poverty of a large part of humanity."
The pontiff said that fasting and almsgiving, along with prayer, creates the proper spirit to enter "into the austere and prayerful climate of Lent."
The 40-day period of Lent includes Good Friday, when Christians recall Christ's crucifixion, and ends with the joyful celebration of Easter.