The head of the Catholic Church, Pope Francis on Wednesday traveled to Lisbon to gather with a million youngsters from across the world at a major Catholic festival, as the Church reflects on its future.

The Argentine Jesuit, whose direct and spontaneous style has proved highly popular with young people, is expected to address youth-focused themes, from ecology and social justice to the war in Ukraine.

The five-day event, called World Youth Day, comes as Francis attempts to galvanise young Catholics at a time when secularism and priest paedophilia scandals cause some to abandon pews in Europe.

The pope, who is in increasingly fragile health and now uses a wheelchair or walking stick to get around, is expected to arrive at Lisbon's military airport around 10:00 am (0900 GMT).

He will then sit down with conservative Portuguese President Marcelo Rebelo de Sousa at Belem Palace.

Meanwhile, organisers expect a million pilgrims from over 200 countries for the week of festive, cultural and spiritual events, which opened Tuesday with a mass on top of a hill overlooking the city and the Tagus river.

Pilgrims carrying flags throng the streets of the capital, where roads are hung with banners and posters featuring the event's yellow, green and red logo.

Access24 reports that the 86-year-old underwent major abdominal surgery just two months ago, but has not stopped an event-packed 42nd trip abroad, with 11 speeches and around 20 meetings scheduled.