In addition to young people from Bosnia and Herzegovina and Croatia, the largest groups came from Italy, Poland, Hungary, Spain, Germany and Austria. Many also came from the United States, France and Ukraine.
The special apostolic visitator for Međugorje parish, Archbishop Aldo Cavalli, read out a letter from the Pope, inviting young people to be living witnesses to the faith by following the example of the Blessed Virgin Mary.
The opening mass was led by Cardinal Juan José Omella, the Archbishop of Barcelona, who said he was pleased to see that so many young Catholics in Europe were not afraid to show their Christianity in an increasingly secularised Europe.
Over 50,000 young people are expected to attend the event over the next five days, the organisers said.
The Međugorje phenomenon dates back to 1981 when six local children claimed that the Virgin Mary revealed herself to them. Some of them say that she still does. The Holy See has looked into their reports but has not yet taken an official position.