A woman threw herself at Pope Benedict XVI and dragged him to the ground as he entered St Peter's Basilica to celebrate Christmas Eve mass on Thursday.
Video footage showed the woman, wearing a red sweatshirt, leaping over a security barricade and rushing at the 82-year-old pope as he began leading the traditional procession to the altar bearing a gold cross.
As a security guard tried to overpower her, the woman succeeded in grabbing Benedict's vestments near the neck and pulled him to the floor, according to the footage which showed several other people falling over in the melee.
The pope was unharmed and back on his feet within moments, and went on to celebrate the mass undaunted by the assault.
Vatican spokesman Federico Lombardi described the woman as "apparently unbalanced" and said she tried to approach Benedict on the same occasion a year ago without getting past the security barrier.
Prominent French Cardinal Roger Etchegaray, 87, broke a leg in the incident though he was several metres (yards) from the pope, Lombardi told AFP, adding that the prelate was rushed to hospital.
Lombardi sought to play down the incident, praising Benedict's "great self-control and control of the situation."
He added: "It was an assault, but it wasn't dangerous because she wasn't armed."
The woman was arrested and questioned by the Vatican police, the ANSA news agency reported.
She appeared confused and agitated, perhaps suffering from mental problems, and said she wanted to hug the pontiff, it added.
Papal security has been tightened since Mehmet Ali Agca, a Turk, shot and nearly killed Benedict's predecessor John Paul II in St Peter's Square in May 1981, but the Christmas Eve procession is one event where tourists and pilgrims can get close to the pontiff.
Dressed in gold and white vestments and mitre, the pope showed no discomfort as he read out his Christmas Eve homily, decrying selfishness, which he said "makes us prisoners of our interests and our desires that stand against the truth and separate us from one another."
The spiritual leader of the world's 1.1 billion Catholics said in Italian: "Conflict and lack of reconciliation in the world stem from the fact that we are locked into our own interests and opinions, into our own little private world."
Thursday's incident occurred less than two weeks after a man with a history of mental problems attacked Italian Prime Minister Silvio Berlusconi at a political rally in Milan, breaking his nose and knocking out two of his teeth.
The attack on the pope took place amid concern over his health after a Vatican decision to schedule the mass two hours earlier than the traditional midnight hour due to the pontiff's advanced age.
Lombardi insisted that the change, a Vatican first, was only a "sensible precaution" for the octogenarian pontiff.
The decision was taken several weeks ago and Lombardi said it was "no cause for alarm," adding that the German pontiff's condition was "absolutely normal" for a man of his age.
Lombardi said the move was aimed at making Christmas "a little less tiring for the pope, who has many engagements during this time".
On Friday, Pope Benedict is to deliver his traditional "urbi et orbi" (to the city and the world) message broadcast to dozens of countries at noon on Friday.
Benedict has had no notable health problems since his 2005 election apart from a fractured wrist from a fall in July while holidaying in northern Italy.
Four years before he became pope however, then Cardinal Joseph Ratzinger spent nearly a month in hospital following a brain haemorrhage, according to the German daily Bild. It said he has suffered from fainting spells.
Pope Benedict's long-serving predecessor John Paul II insisted on observing the tradition of beginning the mass at midnight despite years of ill health, notably the ravages of Parkinson's disease, at the end of his life.
He died in April 2005 aged 84.