Pope Tells 1.5 Million Youngsters to Spread Gospel

* Pope says 'authentic' values important in modern society

* Mass at aerodrome highlight of World Youth Day festival

* Several injured in Saturday storms during prayer vigil

* Protests continue over papal visit

By Judy MacInnes and Sonya Dowsett

MADRID, Aug 21 (Reuters) - Pope Benedict urged 1.5 million young Catholics at a Sunday mass in an aerodrome near Madrid to spread the gospel to others seeking better values than modern society offers.

"We cannot encounter Christ and not want to make him know to others. So do not keep Christ to yourselves! Share with others the joy of your faith," he said in a sermon as part of the activities of the Roman Catholic Church's World Youth Day.

The pope, closing a four-day trip to Spain which has been marked by protests, told pilgrims from around 190 countries that they have been given "the extraordinary task" of being missionaries of Christ in other countries filled with young people looking for better values.

"And, because their heart tells them that more authentic values do exist, they do not let themselves be seduced by the empty promises of a lifestyle which has no room for God."

Hundreds of thousands of young people, bearing flags from their countries and wearing hats to protect themselves from the sun, prayed, applauded and cheered during the service.

The pope read his sermon seated on a white throne under a stylised golden tree, with Spain's King Juan Carlos and Queen Sofia looking on. A choir and orchestra dressed in blue and adorned with white lilies accompanied the mass.

PRAYER VIGIL

Benedict led a prayer vigil at the Cuatro Vientos aerodrome on Saturday evening, but heavy rain prevented him from reading a prepared speech in which he had made a veiled criticism of Spain's legalisation of gay marriages and abortion.

An electric storm followed a day of sweltering heat in Madrid, and wind during the night damaged tents, injuring seven people, emergency services said.

Emergency services helped more than 1,500 young people during Saturday's vigil, most with heat-related complaints.

But hundreds of thousands again packed the aerodrome on Sunday to join the Mass, the highlight of the WYD jamboree.

"When you're at home you just think you're alone in your faith, but when you pray with over a million people it just changes things," said Katharina Eisen, 18, from Germany.

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For an Interactive timeline on recent Papal visits please click on http://link.reuters.com/vud33s

For a FACTBOX on Catholicism in Spain

For at TIMELINE on Benedict as Pope

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The cost of the pope's trip to Madrid has angered some Spaniards struggling with recession and high unemployment. Heavy security has surrounded the pontiff, with roads cut off to traffic and thousands of police on the streets.

World Youth Day organisers say the pope's visit costs the taxpayer nothing. Critics have put the cost at about 100 million euros, mainly in security expenses.

Benedict's trip has reignited criticism by Los Indignados (The Indignant Ones), a group whose mainly young members occupied Madrid's Puerta del Sol square in May to protest against high unemployment and government spending cuts. (Reporting by Judy MacInnes and Sonya Dowsett; additional reporting by Brenton Cordeiro; editing by Alistair Lyon)