More than a million young people who camped out at a Spanish air base for Pope Benedict XVI's final Mass at the church's youth festival awoke to sunny skies today, hours after a fierce thunderstorm forced the pope to cut short his remarks and damaged some structures.

Six people were slightly injured when a tent collapsed during the storm and some makeshift chapels set up on the field's perimeter were also damaged, forcing organisers to announce over loudspeakers this morning that not everyone would be able to receive Communion during the Mass.

Earlier yesterday before the storm hit, hundreds were treated for heat-related problems amid temperatures that seared to 40C.

Despite the discomfort, pilgrims who spent the night in sleeping bags, tents and under tarps seemed unfazed and organisers announced they were opening a new area at the air base to accommodate late arrivals.

Benedict arrived at the field this morning and processed through the crowd in his white pope mobile to the cheers and chants of the flag-waving pilgrims.

This is Benedict's third World Youth Day, the gathering of young Catholics from around the world once every three years that was launched a quarter-century ago by Pope John Paul II in a bid to reinvigorate and spread the faith among the young. It has the feel of a week-long rock concert and camping trip, with bands of flag-toting pilgrims roaming through Madrid's otherwise empty streets to take part in prayer and education sessions, Masses, cultural outings and papal events.

At the end of today's Mass, Benedict will officially announce that the next World Youth Day will take place in Rio de Janeiro in 2013 - a year early to avoid conflicts with the 2014 World Cup in Brazil.