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(Sergio Perez / Reuters)

World Youth Day, the Catholic Churches largest gathering of young people, kicked off in Madrid, Spain on Monday, August 15.

The six day festival draws 800 Bishops, 8,000 priests and nearly one million people from across the world to attend mass, sit in on lectures, meet new people, and, most importantly, receive a visit from the Pope.

Yet, Pope Benedict XVI's arrival on Thursday has been a cause for concern this year. On Tuesday, Spanish police arrested a Mexican chemistry student for allegedly plotting to gas anti-Pope protesters at the festival.

The suspect planned to attack a protest march with "asphyxiating gases and other chemical substances," police said. A police official said the unnamed suspect arrested was a student specializing in organic chemistry.

The police reached the suspect's apartment and found an external hard-drive and two notebooks with chemical equations that had nothing to do with what he was studying and a PC "allegedly used to recruit on the internet".

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A demonstrator protests against special reductions on public transport for visiting pilgrims on the first day of the World Youth Day. (Andrea Comas / Reuters)

More than 100 groups opposed to the Pope's visit plan to protest on Wednesday evening. Many of the groups are part of Spain's M-15 "indignant" movement, that was launched on May 15 against the management of the economic crisis.

The protesters are speaking out against the ostentatiousness of the Pope's visit and the Catholic Church itself.

"We are not angry about the Pope's visit, which some will agree with and others won't, but rather over the financing of it with public money, especially at a time when many services are being cut because it's necessary to curb government spending," 15-M said in a statement.

The Spanish government is spending an estimated 50 million euros ($72 million) to stage the Papal visit; a sum protesters cannot justify given Spain's continuing economic crisis and an unemployment rate of 21%.

Event organizers believe the festival will generate about 100 million euros ($144 m) for the struggling economy, but protesters say that the government will barely break even by granting tax breaks to corporate sponsors and perks like discount subway and bus tickets for attendees.

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Pilgrims receive communion as they attend World Youth Day's opening mass at Madrid's Cibeles Square (Susana Vera / Reuters)

Pope Benedict is still scheduled to arrive on Thursday for his four day visit, but security has been heightened, with constant police surveillance on horseback and foot. Anti-Pope protests are still slated to roll through Madrid's main thoroughfare under the banner "The Pope's visit, not with my taxes."

The march also protests issues like gay marriage and separation of Church and State. Over 200 gay and lesbian protesters have organized a "kiss-in" when the Pope arrives on Thursday.

As expected, not everyone is OK with the demonstrations. Some Roman Catholic groups like Hazte Oir (Make Yourself Heard) have tried to ban the protests with a petition describing it as "an expression of intolerance and religious hatred."