ROME, April 18 (Xinhua) -- Mass surveillance would never prevent terrorism fully, American whistleblower Edward Snowden said at the International Journalism Festival in Perugia on Friday, local media has reported.

Snowden was welcomed as a chief guest by the audience attending the third day of the festival, which has gathered journalists and experts from all over the world since 2006.

"Even the most extensive monitoring system would never be able to make us perfectly safe from terrorism," Snowden said, while taking part in a debate via video link, which was broadcasted by Ansa News agency.

"Yet, mass surveillance is often used by intelligence agencies to spy on citizens regardless if a crime is being committed or not," he added.

Snowden long served the National Security Agency (NSA), the Central Intelligence Service (CIA) and other American security agencies as technology and cyber-security expert.

In 2013, he revealed mass surveillance techniques were used by NSA to spy on millions of citizens, businesses and politicians, and his disclosures sparked tensions between the United States and many of its allies.