Pope Francis arrives in Lisbon on Wednesday for a global gathering of young Catholics taking place in the shadow of Portugal's huge clergy sexual abuse scandal and criticism of soaring costs for the event. Hundreds of thousands of young people from around the world have descended on Lisbon to welcome Francis, who is due to land at Lisbon's military air base at 10 a.m. (0900 GMT). World Youth Day was devised by the late Pope John Paul for young Catholics in their teens or early 20s and is held every two or three years in a different city.