Spanish police say they have arrested a Mexican chemistry student who was allegedly planning to attack demonstrators who are protesting an upcoming visit to Spain by Pope Benedict.

Officials say the student, whose identity has not been released, was arrested Tuesday in Madrid.

Police say he planned to use asphyxiating gases and other chemical substances to attack protesters, who are holding a demonstration on Wednesday night in downtown Madrid to protest the use of taxpayer money to pay for the Pope's visit.

The Pope arrives in Spain on Thursday for a four-day visit, during which he will celebrate the Catholic Church's World Youth Day with thousands of pilgrims.

Protest organizers say they are not against the Pope's visit, but only against the use of public funds to pay for the visit, which officials say will cost about $72 million.