The ceremony was attended by around 400,000 Roman Catholics who have gathered from around the world for the festival.
It was led by the Archbishop of Rio, Orani Joao Tempesta.
Around 1.5 million young pilgrims are expected to attend World Youth Day.
Pope Francis is due to attend World Youth Day on Thursday, a highlight of the five-day event.
It is his first international trip since becoming head of the Roman Catholic Church in March this year.
At the official ceremony welcoming him to Brazil, the Pope - who hails from neighbouring Argentina - noted the meaningfulness of the first international trip of his pontificate being Latin America.
In his address, he described young people as "the window through which the future enters the world".
The Pope will lead Holy Mass at the close of World Youth Day on Sunday.