Pope Francis on Thursday urged young people to focus on caring for the planet and fighting climate change, calling for an "integral ecology" that melds environmental protection with the fight against poverty and other social problems.

The 86-year-old Francis has made the protection of the environment a cornerstone of his pontificate, noting in his landmark 2015 "Laudato Si" (Praised Be) encyclical that the planet was "beginning to look more and more like an immense pile of filth".

"We must recognise the dramatic and urgent need to care for our common home. Yet this cannot be done without a real change of heart," he said during an open-air address to students at Lisbon's Catholic University on the second day of his visit to Portugal.

"We cannot be satisfied with mere palliative measures or timid and ambiguous compromises," he added, speaking in his native Spanish.

The audience of around 6,500 people, which included Portuguese President Marcelo Rebelo de Sousa and bishops, cheered and applauded as the pontiff took the stage while a choir sang "Jesus Christ you are my life".

The leader of the world's 1.3 billion Roman Catholics said: "Keep in mind that we need an integral ecology, attentive to the sufferings both of the planet and the poor. We need to align the tragedy of desertification with that of refugees, the issue of increased migration with that of a declining birth rate".

"Instead of polarised approaches, we need a unified vision, a vision capable of embracing the whole."

The first Latin American pope arrived in Lisbon on Wednesday for World Youth Day festivities, a week-long international Catholic jamboree that is expected to draw one million people.

Francis, who met with clergy and victim of clerical sexual abuse on Wednesday, will deliver a Mass on Sunday in Lisbon on the last day of his five-day visit to Portugal.

World Youth Day, created in 1986 by John Paul II, is the largest Catholic gathering in the world and will feature a wide range of events, including concerts and prayer sessions.

This edition, initially scheduled for August 2022 but postponed because of the pandemic, will be the fourth for Francis after Rio de Janeiro in 2013, Krakow in 2016 and Panama in 2019.