A Mexican student was arrested in Madrid on Tuesday after posting his intention to attack anti-papal protestors with toxic gases, including sarin, on the internet.

A staunch Catholic, the man said that he could not allow protests against the pope. Benedict XVI is due to arrive in Madrid on Thursday ahead of a weekend of events to mark World Youth Day 2011. Hundreds of thousands of Catholic faithful have already descended on the capital. The pontiff is scheduled to attend a welcome ceremony in the downtown Cibeles area on Thursday evening and to hold a Mass at the Cuatro Vientos aerodrome on Saturday night.

World Youth Day 2011 officially began on Tuesday, with a Mass in Cibeles presided over by the archbishop of Madrid, José Antonio María Rouco Varela. World Youth Day runs until Sunday August 21.

More information Gas attack plotter was a papal visit volunteer worker

Since Tuesday, many of the city's main roads have been closed off due to the papal visit.