The pope slipped and fell to the ground but then resumed the service, a Vatican spokesman said.
Father Federico Lombardi said the woman appeared to be "unstable." She was taken away by Vatican security.
The pope was helped back to his feet and, apparently unharmed, continued the service, Father Lombardi said.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.
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