The woman had just delivered a first reading from the Book of Isaiah, and was walking down the steps with a fellow reader. A female member of the congregation can be seen waiting innocuously by the pews; but she suddenly lashes out with her fist in an unprovoked attack, pummeling her twice in the face.
Audible gasps of witnesses can be heard above the playing organ; one of the shocked priests carrying incense stops in his tracks after watching the whole thing happen, before continuing on with the ceremony.
A motive for the attack is unclear. Neither reader — nor any of the priests — were wearing masks, but the attacker was.
The victim was checked but did not require medical attention.
"We immediately followed up on it off-camera and it's important for us to do that because people are at home and continuing to participate in Mass," Rev. Dennis Gill of the Cathedral Basilica told Action News 6 ABC.
"Nonetheless, it was very upsetting. Parishioners attending in person were shocked, as were so many that watch the 11 a.m. Mass live-streaming online."
Police told the station the woman, who has been seen at the church before but is not a parishioner — is not in custody.
In a Facebook post, Philadelphia Archbishop Nelson Perez called it a "senseless act of aggression."
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"Such behavior is unacceptable at all times, especially within the confines of a church building and during the celebration of the Holy Mass. I was saddened to learn of this incident and regret that it took place," he said.
He called for prayers for everyone involved — including the attacker — adding: "Violence has no place in our world and every life is a precious gift from God."