Many had waited for hours in the scorching sun, with temperatures in Lisbon rising to close to 36 degrees on Saturday according to Portugal’s meteorological institute, to take part in the vigil led by the pope.
The papal visit was on the occasion of a Catholic youth event held every three years, each time in a new city.
Pope Francis will also return to the park on Sunday morning for a final service before returning to Vatican City.
Earlier on Saturday, the Pope visited the Fatima shrine, a site north of Lisbon dedicated to the Virgin Mary, where he was welcomed by around 200,000 people.