Pope Francis has closed an international festival of Catholic youth with a huge outdoor Mass and his own "I have a dream" speech, saying it was for world peace, especially for Ukraine.
About 1.5 million people attended his closing Mass at a riverside park in the Portuguese capital, the Vatican said, quoting local authorities.
Many of the faithful slept outdoors, having attending a vigil there last night, and they gathered in sweltering heat.
Speaking after the Mass, the 86-year-old Pope Francis urged the young people to take the fraternal experiences of the six-day event back home and apply them to their daily lives.
"Dear friends, allow me, this old man, to share with you young people a dream that I carry within me: it is the dream of peace, the dream of young people praying for peace, living in peace and building a peaceful future," Pope Francis said.
"As you return home, please continue to pray for peace. What is more, you are a sign of peace for the world, showing how different nationalities, languages and histories can unite instead of divide. You are the hope of a different world," he said.
He asked them to think of the young people who could not come to the event because of the world's many armed conflicts and wars, adding: "In thinking of this continent, I feel great sorrow for beloved Ukraine, which continues to suffer greatly".
Pope Francis, who is returning to Rome this evening after an event to thank volunteers at the World Youth Day festival, met a delegation of 15 young people from Ukraine during his trip.
This was his first foreign trip since he spent nine nights in hospital after undergoing hernia surgery in June.
12 Irish bishops, along with more than1,500 pilgrims from throughout the island, are in Lisbon for the World Youth Day gathering.
Last night the app of the World Youth Day sent heat warnings reminding people to drink water and apply sunscreen.
A tractor with a water tank sprayed the crowd and a succession of bands and speakers from a huge stage with the backdrop of a cross tried to keep the participants' minds off the heat at an event that has been dubbed the Catholic Woodstock.
Today's Mass was concelebrated by 700 bishops and 10,000 priests, who distributed communion to the huge crowd.
The pope announced that the next World Youth Day would beheld in Seoul, South Korea in 2027.
One of the recurring themes of the pope's visit was social media and its potentially negative effects on young people.
During the week, Pope Francis urged them to beware the false happiness lurking in the virtual world and at another event, the young people themselves reflected on their anxieties, enslavement to the "tyranny" of social media and yearning to save the planet.
The trip took place in the shadow of a report six months ago by a Portuguese commission said at least 4,815 minors were sexually abused by clergy - mostly priests - over seven decades in the country.
It was just one in a series of reports around the world that have exposed clerical sex abuse and rattled the Catholic Church in recent years.
On Wednesday Pope Francis said the Church needs a "humble and ongoing purification" to deal with the "anguished cries" of victims of clerical sexual abuse, and met privately with 13 victims.
