The week-long faith extravaganza of World Youth Day is under way in Madrid, with more than one million young Catholics from around the world expected to pour into the city before Pope Benedict arrives on Thursday.

Highlights of the celebration include a mass confession and prayer vigil in the presence of the Pope in Madrid Saturday and a celebration of Mass on Sunday morning. It will be the Pope's third visit to Spain since his election in 2005, but comes at a time when the nation's economy is faltering and bitter debates are raging over the cost of the festival and the Catholic church's role in Spanish politics.

Upon arrival Thursday, Spain's royal family will greet Pope Benedict at Madrid's Barajas airport. The Pope will also meet with Spanish Prime Minister Jose Luis Rodriguez Zapatero, whose government has promoted several policies opposed by the church, including relaxing Spain's divorce laws, easing restrictions on abortion, legalizing same-sex marriage and allowing gay couples to adopt children.

Left-leaning groups are planning a major protest march Wednesday on the eve of the pope's arrival. Criticism of the papal visit has come mainly from the “Indignant Ones” protest movement, who have been protesting the country's economic woes, government spending cuts, and 21 percent unemployment.

Many are also unhappy with the cost of the pope's visit, which is estimated at more than $72 million. They feel this spending is unnecessary when the country is undergoing a deep economic crisis.

On Tuesday, Spanish police arrested a Mexican chemistry student who was allegedly planning to attack demonstrators who are protesting the Pope's visit.

The authorities say he was planning to use asphyxiating gases and other chemical substances to attack protesters, who are scheduled to hold a demonstration Wednesday night in downtown Madrid to protest the use of taxpayer money to pay for the Pope's visit.