The announcement that the pope would not speak at the opening of the university's academic year on Thursday, Jan. 17, came after street protests by anti-clerical students and professors on Tuesday afternoon.

It is the first cancellation of an appearance by the pontiff in the face of hostility since his election in April 2005.

"Following the well-known events of these days...it was considered appropriate to cancel the event," the Vatican said in a statement.

Benedict, who had been scheduled to deliver a speech at the start of La Sapienza's academic year, would still send his text to the university, the statement said.

Escalating dissent

While tensions around the pontiff's visit had escalated throughout the day with some 100 students staging a sit-in at La Sapienza's main hall, the Vatican's evening announcement came as a surprise.

Earlier on Tuesday afternoon, Giovanni Maria Vian, the editor of the Vatican's mouthpiece newspaper L'Osservatore Romano, lamented the controversy stirred by the pontiff's planned visit, but predicted it would go ahead.

And university rector Renato Guarini said that while opponents of the visit would be allowed to hold a demonstration on campus on Thursday, the pontiff's invitation was still valid.

"The Pope is a man of culture and great thinker" who should be allowed to have his say, Guarini said in an interview with private news television news channel SkyTG24.

Academics oppose Pope's stance

The opposition to the planned visit began when a group of 67 academics signed a letter requesting that Guarini withdraw the invitation to the Pope.

In the letter, the academics said they opposed the pontiff's "incongruous" visit to a secular university, and called on the university to revoke the "disconcerting invitation" to Benedict, whom they accuse of being a reactionary and an opponent of free thought and research.

Many scientists fault the intellectual, conservative and tradition-minded pope for a series of positions he has taken that they say subordinate science and reason to faith.

The faculty protest against the visit was spearheaded by a well-known physicist, Marcello Cini, a professor emeritus at La Sapienza.

Pope "is playing the victim"

On learning of the cancellation, Cini told AFP news service, "I'm satisfied, that's what I was asking for. I thought, and I continue to think, that this visit was ambiguous and an attack on the independence of culture and the university."

He added: "By cancelling, he is playing the victim, which is very intelligent. It will be a pretext for accusing us of refusing dialogue."

In their letter, the academics cited a 1990 speech made by Benedict when he was still a cardinal, in which he allegedly justified the Catholic Church's actions against Galileo, the 17th-century physicist who ran afoul of Church doctrine by insisting that the Earth orbits the sun.

During the 1990 speech delivered in Parma, Italy, the then-Cardinal Joseph Ratzinger had quoted Austrian-born philosopher Paul Feyerabend as saying that the church's 1633 heresy trial against Galileo had been "reasonable and fair." In court, Galileo was forced to recant his theories.

Students focus on Galileo comments

Benedict "condemns centuries of scientific and cultural growth by affirming anachronistic dogmas such as creationism, while attacking scientific free-thought and promoting mandatory heterosexuality," students opposed to the visit, using the name Physics Collective, said on their Web site.

The students opposed to the visit kicked off an "anti-clergy week" on Monday by showing a film on Galileo.

On Monday, Vatican Radio said attempts to prevent Benedict from speaking at the campus amounted to censorship.

Catholic and conservative politicians have condemned the university's actions as a sign of intolerance, while opinions in the country's governing center-left have been split, with some saying the pontiff's opponents have the right to show their dissent.