ROME — It’s a big deal when the pope agrees to speak at an event that isn’t church-related.

It’s an even bigger deal when public protest forces him to cancel.

Veteran Vatican-watchers said they had never seen anything like it. Pope Benedict XVI yesterday abruptly called off plans to speak at Rome’s prestigious La Sapienza university, after students and professors rallied to proclaim him pontiff-non grata.

More than 60 professors signed a letter to the public school’s rector saying the pope’s appearance, which had been scheduled for the opening of the academic year tomorrow, was an affront to people of science and to the “secular” nature of the institution.

Students staged a sit-in yesterday, waving banners scrawled with angry slogans (“Knowledge needs neither fathers nor priests”) and launching what they dubbed “anti-cleric week.”

“This pope unfortunately is not particularly friendly to science,” a physics professor, Andrea Frova, one of the La Sapienza academics who signed the petition, said in an interview.

Mr. Frova and the others said they were offended by a comment made by the pope 17 years ago when he was Cardinal Joseph Ratzinger, that called the heresy trial of 17th-century astronomer Galileo “reasonable.” (In fact, Ratzinger was quoting another philosopher in that passage, part of a long speech on the Roman Catholic Church and Europe.)

The issue goes beyond Galileo, Mr. Frova said, and to the church’s position today on stem-cell research, evolution, and genetic engineering.

“History changes, the scientific problems are different today than in the time of Galileo, but the attitudes of the church stay the same,” he said.

The pope would be welcome at the university to debate these issues, Mr. Frova said, but not to deliver a speech in which there would be no opportunity for discussion or response.

Vatican officials initially said the pope would go forward with the appearance and that the protests smacked of censorship.

But the prospect of ugly public displays apparently changed minds at the Vatican. Given the incidents of the last few days, a brief statement said, “It was considered opportune” to scrap the event.

Is the pope hostile toward science? Benedict is an intellectual who has emphasized the importance of reason in the practice of faith.

Yet he also says evolution is the work of a divine creator, and he has helped defeat Italian laws that liberalized scientifically assisted fertility.