Thousands of leading academics, alarmed by an executive order signed by President Donald Trump on Friday afternoon instituting “extreme vetting” of refugees with a goal of keeping radical Islamic terrorists from entering the country, have signed onto a petition denouncing his action.

The executive order begins a 30-day ban on allowing people from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen from entering the United States. It suspends the U.S. Refugee Admissions Program for 120 days.

“I’m establishing a new vetting measure to keep radical Islamic terrorists out of the United States of America,” Trump said when he signed it. “We don’t want them here. We want to make sure we are not admitting into our country the very threats our soldiers are fighting overseas.”

Some welcomed what they saw as a strong signal that the United States would work to exclude people intent on doing harm.

But at campuses across the country in recent days, many people reacted to the prospect of “extreme vetting” with alarm and protests, such as a rally at Harvard University on Friday evening.

By Friday evening, 11 Nobel laureates and thousands of other academics – many well-known scholars, including Fields Medalists, John Bates Clark medalists, members of the National Academy of Sciences and at least one MacArthur Fellow – had signed on. By early Saturday, a 12th Nobel laureate had joined the list.

They were getting about 10 emails a minute, and 15 to 20 volunteers were working to add signatures, an organizer of the effort said Friday evening.

What’s at stake, said Emery Berger, a professor in the College of Information and Computer Sciences at the University of Massachusetts Amherst, who is not one of the organizers of the petition but supports the effort, begins with free exchange of information.

But there’s more. He said he has already heard academics overseas planning to avoid, or boycott, conferences in the United States. “It’s very chilling,” he said.

Students are horrified, he said, at the prospect of not being able to get back to their U.S. university if they return to their home country.

“I’m sure it will send really promising star students across the border to Canada or elsewhere,” Berger said. The order comes just as many U.S. universities are offering admission to overseas students for the next academic year. He said an administrator told him Friday to get admissions offers to top Iranian students “yesterday.”

Several Trump administration officials did not immediately respond to requests for comment Friday evening.

Trump has repeatedly called for more stringent limitations on immigration to the United States, especially after a terrorist attack in December in San Bernardino, California.

After an attack at Ohio State University in November, Trump tweeted that “ISIS is taking credit for the terrible stabbing” and that Abdul Razak Ali Artan, the Ohio State student who drove a Honda sedan through a crowd outside a school building before slashing at people with a butcher knife, was “a Somali refugee who should not have been in our country.”

The petition against Trump’s order, with its growing list of academics, reads, in part:

“This Executive Order is discriminatory. The EO unfairly targets a large group of immigrants and non-immigrants on the basis of their countries of origin, all of which are nations with a majority Muslim population. This is a major step towards implementing the stringent racial and religious profiling promised on the campaign trail. The United States is a democratic nation, and ethnic and religious profiling are in stark contrast to the values and principles we hold.”