While the Vatican denied the unarmed man intended to harm the Pope, it did say that the 26-year-old German citizen showed "clear signs of derangement" while attempting to climb aboard the vehicle, the Italian news agency ANSA reported.
"He was just trying to draw attention to himself," the Vatican said.
Yet with the incident drawing comparisons to the 1981 assassination attempt on Pope John Paul II, the bodyguards took no chances in restraining the man.
That attack, also in St. Peter's Square, on the previous pope left the spiritual leader seriously wounded. Turkish citizen Mehmet Ali Agca was convicted of the assault.
ANSA reported the assailant in Wednesday's incident was taken to a regional psychiatric hospital.