By Philip Pullella and Catarina Demony
LISBON (Reuters) -Pope Francis closed an international festival of Catholic youth on Sunday with a huge outdoor Mass and his own "I have a dream" speech, saying he longed 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 on Saturday night, and they gathered in sweltering heat.
Speaking after the Mass, the 86-year-old Francis urged the young people to take the fraternal experiences of the six-day jamboree 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," 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".
Francis, who returned to Rome on Sunday evening after an event to thank and pray with 25,000 volunteers at the World Youth Day festival, met a delegation of 15 young people from Ukraine during his trip.
In his speech to volunteers, Francis referred to the Portuguese seaside town of Nazare, which has some of the biggest waves in the world, and described them as "surfers of love".
The TAP airline plane carrying the pontiff, his entourage and reporters took off from Lisbon's Figo Maduro military air base shortly after 6:20 p.m. (1720 GMT). Bishops and Portuguese leaders lined up to say farewell to Francis before he boarded the plane.
TEN THOUSAND PRIESTS ON HAND
Sunday's Mass was concelebrated by 700 bishops and 10,000 priests, who distributed communion to the huge crowd.
Marina Sylvester, 22, from the pope's native Argentina, was one of hundreds of thousands of young people who spent the night in the riverside area.
She woke up at dawn and by 7 a.m. was showing off her dance moves as a well-known Portuguese priest DJ, Guilherme Peixoto, played upbeat songs. "It has been one of the best experiences of my life," she said.
The pope said the next World Youth Day would be held 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, Francis urged them to beware the false happiness of the virtual world and at another event, the young people reflected on their anxieties, their yearning to save the planet and the "tyranny" of social media.
The trip took place in the shadow of a report six months ago by a Portuguese commission that found at least 4,815 minors were sexually abused by clergy - mostly priests - over seven decades in the country.
It was one of a series of reports around the world that have exposed clerical sex abuse and rattled the Catholic Church in recent years.
Francis said on Wednesday 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.
(Reporting by Philip PullellaEditing by Frances Kerry and Barbara Lewis)