Pope Benedict XVI on Saturday urged priests to observe celibacy and to "leave behind all earthly desire," at a mass with future priests in the Spanish capital Madrid.
Priests needed to live "in celibacy" and "in austerity of life," said the head of the Catholic Church, which has come under widespread criticism over child sex abuse scandals among its priests, DPA reported.
"Approach the priesthood only if you are completely determined to exercise it in obedience to the church's precepts," the pope, who was visiting Madrid on occasion of the World Youth Day celebrations, said in his homily.
Benedict also indirectly stressed the need for priests to oppose abortion, saying they should be "messengers of the supreme dignity of the human person and therefore its unconditional defenders."
Future priests from five continents attended the mass at Almudena Cathedral.
In Spain, the vocation of priesthood is on the decline, with the average age of priests around 65. Some analysts attribute the lack of priests partly to the requirement of celibacy.
Prior to the mass, the pope absolved four young people from their sins at Madrid's Retiro park, where 200 confessionals have been set up. Thousands of pilgrims have confessed their sins to priests from all over the world in more than 50 languages.
About one million pilgrims from 190 countries are estimated to have come to the Spanish capital for the pope's four-day visit, which will culminate in a gigantic open-air mass on Sunday.
The pope drew hundreds of thousands of people to his appearances at a welcoming event on Thursday and a Way of the Cross - a series of prayers reflecting on Jesus' suffering before he was crucified - on Friday.
The visit has also sparked criticism, especially over its estimated cost of 50 million euros (70 million dollars) at a time when Spain has a 20-per-cent unemployment rate.
Protest rallies have taken place on three nights, with police preventing demonstrators from clashing with pilgrims. Several hundred people demonstrated against the pope's visit and what they described as police brutality on Friday evening.