The controversy was unparalleled in a country where criticism of the Roman Catholic church is normally muted. The Pope had been due to speak tomorrow during ceremonies marking the start of the academic year at Rome's largest and oldest university, La Sapienza. But the Vatican said last night it had been "considered opportune to postpone" his visit.

The announcement followed a break-in and sit-in at the rector's office yesterday by about 50 students and a furious row over a letter signed by more than 60 of La Sapienza's teachers, asking that the invitation to the Pope be rescinded.

The signatories of the letter said Benedict's presence would be "incongruous". They cited a speech he made at La Sapienza in 1990, while he was still a cardinal, in which he quoted the judgment of an Austrian philosopher of science who wrote that the church's trial of Galileo was "reasonable and fair".

The letter said: "These words offend and humiliate us." Among the signatories was the physicist Prof Luciano Maiani, who was recently appointed to head Italy's main scientific research body, the Consiglio nazionale delle ricerche.

Maiani said he had later dropped his opposition to the visit after learning that the Pope would not be making the keynote address. But the daily La Stampa reported that a number of foreign scientists had since added their names to the initiative.

One students' group declared an "anti-clerical week" to protest at the Pope's presence. Among numerous banners and placards put up around the campus, there was one that read: "Galileo recanted. We shall hold out against the papacy."

The Pope is known for his deeply conservative outlook and the controversy is the latest in a string of rows since his election three years ago. He upset Muslims with another quotation in an academic lecture, on that occasion from a medieval Byzantine emperor.

He has since been criticised by Latin Americans for his views on the colonisation of their continent and by Protestants for saying their denominations ought not to be considered as churches.

The newspaper Il Giornale, which republished his 1990 speech, said the Pope had "expressed a different position" from that of the Austrian scholar Paul Feyerabend, "absolutely not adopting it as his own". The Vatican's own daily, L'Osservatore Romano, carried an article by the Jewish mathematician Giorgio Israel, in which he wrote that the Pope's address "could well be considered, by anyone who read it with a minimum of attention, as a defence of Galilean rationality against the scepticism and relativism of postmodern culture".

Speaking before last night's announcement, Italy's deputy prime minister, Francesco Rutelli, said: "The attempt to silence [Benedict] in a place that is a forum for study, teaching and dialogue ... seems inconceivable." He noted that a pope had founded La Sapienza in 1303.

However, the trade minister, Emma Bonino, said the Pope already "held the floor morning and night".

Rightwing opposition MPs were outraged. One suggested La Sapienza, which means "wisdom" or "learning" ought now to be renamed La Ignoranza

Backstory

· Galileo Galilei was the Inquisition's most high profile victim. But by recanting his view that the earth moved around the sun, he managed to pay for his defiance of Catholic teaching, not with his life, but his freedom.

· Born in Pisa in 1564, Galileo was a polymathic genius - a physicist, astronomer and mathematician who improved both the refracting telescope and compound microscope

· After ridiculing the views of the then Pope Urban VIII in his Dialogue Concerning the Two Chief World Systems, Galileo was ordered to stand trial for heresy in 1633. The judgment found that his view of the solar system was "absurd, philosophically false, and formally heretical, because it is expressly contrary to Holy Scriptures".

· He spent the rest of his life under house arrest on orders of the Inquisition and died in 1642. It was not until 1835 that his Dialogue was dropped from the Index of banned books. John Hooper