His visit coincides with the church's continuing reckoning with the clergy sexual abuse scandal.
More than one million young people were expected to attend the gathering in Lisbon, a city experiencing high summer temperatures, which prompted Portuguese health authorities and Prime Minister Antonio Costa to urge those attending to drink plenty water and wear hats.
In an interview, Cardinal-elect Americo Aguiar, a Lisbon bishop and an organiser of the event, highlighted the recent challenges of Covid lockdowns, Russia's invasion of Ukraine and global economic uncertainty.
On the Pope's agenda
The Pope met Portuguese officials at the Belem National Palace before a visit to the 16th-century Jeronimos Monastery and church.
During the first speech of his visit, the Pope said Europe should ask itself where it was heading if it is “not showing the world paths of peace, creative ways for bringing an end to the war in Ukraine”.
“For the world needs Europe, the true Europe. It needs Europe's role as a bridge and peacemaker,” he added in his address to officials and diplomats at Lisbon's riverside Belem Cultural Centre.
The Pope said the world was currently "sailing amid storms on the ocean of history", including the tempest of the war in Ukraine, and he urged Europe to find the resolve to help end it and other conflicts.
He said Europe should divert money spent on armaments and use it to boost education and fund family-friendly legislation to help reverse a falling birth rate aggravated by prohibitive costs of housing for young couples.
The pontiff also urged Europe to rise to the challenge of "welcoming, protecting, promoting and integrating" migrants, both for humanitarian reasons and as a means of boosting dwindling populations.
The Pope, who is in increasingly fragile health and now uses a wheelchair or walking stick to get around, then headed to the Holy See's diplomatic mission in Lisbon where he will be staying.
Worshippers shrieked as he passed by in his wheelchair, reaching out to touch his white cassock and take a photo with their mobile phones.
The 86-year-old pontiff had major abdominal surgery only two months ago, but that did not stop his 42nd trip abroad, an event-packed visit with 11 speeches and about 20 meetings scheduled.
He will meet members of the Portuguese Catholic hierarchy, currently grappling with a legacy of sexual abuse.
The Pope is expected to privately meet abuse survivors this week and may address the issue in his public remarks, as he has done in the past.
A recent independent commission reported that at least 4,815 boys and girls had been abused in the country since 1950, leading to widespread criticism of Portuguese bishops for their initial response to these findings.
The bishops were reluctant to suspend active clergy members named in the report and initially resisted paying reparations to victims.
The Portuguese Catholic Church's plan to unveil a memorial to victims during World Youth Day was scrapped a few weeks ago. Instead, victims' advocates launched a campaign called This is our Memorial and placed a billboard in central Lisbon stating “4,800+ Children Abused”.
Pope Francis is expected to address climate change during his visit, an issue he has spoken out about previously in the context of wildfires in Greece.
World Youth Day returns to Europe
World Youth Day, initiated in the 1980s by John Paul II, aims to invigorate young Catholics and their faith.
The event is returning to Europe for the first time since 2016.
Youths from Ukraine and Russia are expected to attend and the war between those countries is to be a focal point when Francis visits the shrine of Fatima, associated with a prophecy about peace and Russia.
Due to the expected high temperatures, additional water tanks and drinking fountains have been installed, while reusable water bottles and sun hats will be handed out.
There is also concern for Pope Francis, who was in hospital for nine days in June. His condition has raised concerns about his ability to withstand the heat.
With hundreds of thousands of young Catholics from all over the world attending, the international festival is a pivotal event for the church and the future generation of Catholic devotees.
There was a cheerful reception upon his arrival, with young people chanting: “We are the Pope's youths.”