VATICAN CITY (AP) - A woman jumped the barriers in St. Peter's Basilica and knocked down Pope Benedict XVI as he walked down the main aisle to begin Christmas Eve Mass on Thursday, a Vatican spokesman said. Vatican security says security measures will be examined in light of the incident.

The Rev. Ciro Benedettini said the 82-year-old pope quickly got up and was unhurt. As the pope's procession was making its way toward the main altar gasps rang out through the public and a commotion could be seen among the clergymen surrounding Benedict. The procession came to a halt and bodyguards rushed to the trouble spot.

Benedettini said the woman who pushed the pope appeared to be mentally unstable and had been arrested by Vatican police. He said she also knocked down Cardinal Roger Etchegaray, who was taken to hospital for a check up.

"During the procession an unstable person jumped a barrier and knocked down the Holy Father," Benedettini told The Associated Press by telephone. "(The pope) quickly got up and continued the procession."

After the incident, Benedict calmly resumed his walk to the basilica's main altar to start the Mass.

Benedict made no reference to the incident as the service started. As a choir sang, he sprinkled incense on the main altar before opening the service with the traditional wish for peace in Latin: "Pax vobis" ("Peace be with you"). The faithful responded: "Et cum spiritu tuo" ("And also with you").

It was the second year in a row there was a security breach at the service. At the end of last year's Mass a woman who had jumped the barriers got close to the pope but was quickly blocked on the ground by security.

Benedettini said it was not immediately known if the same woman was behind Thursday's incident.