Madrid: Hundreds of thousands of sweltering pilgrims crammed into central Madrid for a giant open-air mass, launching a million-strong, six-day Catholic youth party for Pope Benedict XVI.

The rock-festival style celebration of World Youth Day has drawn faithful from 193 nations, according to organisers -- but also criticism in the midst of Spain's economic hardship.

From a vast white stage adorned with an image of Mary and Jesus before City Hall in the emblematic Plaza Cibeles square, Madrid's archbishop, Cardinal Antonio Maria Rouco Varela, celebrated the inaugural mass in the evening.

Some 800 bishops, archbishops and cardinals from around the world joined him, many shielding themselves from the sun with white umbrellas. Another 8,000 priests concelebrated the mass.

The service, accompanied by hundreds of choristers, was dedicated to the late Pope John Paul II, who launched the first festival in 1986 as a way to revitalize the faith among young Catholics.

In homage to John Paul II, a vial of blood taken in the final days of his life in 2005 was placed on a pulpit used in the service, inside a sculpture in the form of an open book.

Railing against "rampant moral and spiritual relativism," the Madrid archbishop urged young people in the 21st century to follow the path of the "humble, simple pilgrim".

The Church opened 200 white confessionals in the form of boat sails along the main thoroughfare through Madrid's Retiro Park, where priests and bishops from across the world listened -- in 30 languages -- to confessions.