Pope Francis has sent a message of "faith and fraternity" to World Youth Day as he prepares to set off to Krakow in Poland to join in prayer and praise with hundreds of thousands of young Catholics.

World Youth Day begins next Tuesday with the final Mass the following Sunday.

Pope Francis says he is looking forward to meeting "the beloved Polish nation", the home country of his predecessor but one, Saint John Paul II, who began the World Youth Day tradition. They take place every three years with the last one in Rio.

In the video message,: "Dear young people of Poland, I know that for some time now you have been preparing, especially with your prayers, for this great encounter in Krakow. I thank you heartily for everything that you have done, and for the love with which you have done it. Even now I embrace you and I bless you."

He continues: "I am very anxious to meet you and to offer the world a new sign of harmony, from many races, languages, peoples and cultures, but all united in the name of Jesus, who is the Face of Mercy."

He praises Poland for perseverance "through the power of faith" during many crises and trials over the last century.

The moral and spiritual "health" of a nation is seen in its families, he says.

"That is why Saint John Paul II showed such great concern for engaged couples, young married couples and families. Continue along this road!"

As many as two million young adult Catholics are expected in Krakow over next week to celebrate World Youth Day with Pope Francis, who will conclude his trip to Poland with a visit to the Auschwitz-Birkenau Nazi death camp.

James Kelliher, youth spokesman for London's Westminster diocese, is attending his third World Youth Day. He met his wife on the last one. Here he talks about the impact the event has had on his life.