A chemistry student, suspected of planning a gas attack against protesters opposing a visit by Pope Benedict XVI, has been arrested by the Spanish police.

The pope is due to arrive in Spain on Thursday for a four-day visit, and a protest march is scheduled for Wednesday evening in Madrid, according to an AP report.

The suspect is a Mexican exchange student studying organic chemistry. The officials did not disclose the name or age of the suspect, who was arrested in Madrid on Tuesday.

According to the police, the suspect planned to attack anti-Pope protesters with "suffocating gases" and other chemicals. Officials said he was in Madrid studying at Spain's top government research body, the Spanish National Research Council.