Thousands of professors and other academics have put their names to an online petition that condemns President Donald Trump’s executive order severely limiting immigration and travel from several countries.

The petition, which had more than 3,000 signatures as of Saturday afternoon, denounces the order as discriminatory and likely to tear families apart. It also warns that the new policy could be disastrous for science and research in the United States.

Trump’s order, signed Friday, halts the U.S. refugee resettlement program for four months and bars people from seven Muslim-majority countries for at least 90 days. The status of affected green card holders, who are legal permanent U.S. residents, will be addressed on a case-by-case basis, officials said.

“US research institutes host a significant number of researchers from the nations subjected to the upcoming restrictions,” the petition states. “From Iran alone, more than 3000 students have received PhDs from American universities in the past 3 years.”

Trump’s action will necessarily limit collaborations between researchers from different nations and could “potentially lead to the departure of many talented individuals who are current and future researchers and entrepreneurs in the US,” the petition says. For those reasons, the petition organizers call the executive order “detrimental to the national interests of the United States.”

Signatories to the petition include 15 Nobel laureates in fields like physics, economics and medicine.

Stephanie Keith/Getty Images Protesters rally at New York's John F. Kennedy International Airport in opposition to Trump's severe restrictions on immigration and travel.

Emery Berger, a professor in the College of Information and Computer Sciences at the University of Massachusetts, Amherst, is not one of the organizers but has signed the petition. He called the executive order “very chilling” in a Washington Post interview.

“I’m sure it will send really promising star students across the border to Canada or elsewhere,” Berger said, referring to those choosing where to study. He also noted that students from the affected countries who are currently in the U.S. are frightened of not being allowed back if they travel out of the U.S.

The executive order has had near-immediate impact on academia and research.

There have been multiple reports of students detained or blocked from returning to the U.S. after traveling abroad, including an anthropology Ph.D. candidate who told The Guardian that he dare not go to his home country of Iran because he has vocally criticized human rights violations there.

Sarah Knuckey, a Columbia Law professor, said the visa ban would impede collaborative research on how to improve people’s health during periods of armed conflict.

One of many harmful effects on education: ban stops @sanaacenter coming to NY to work with us on project to improve health in armed conflict — Sarah Knuckey (@SarahKnuckey) January 28, 2017

ABC’s chief foreign correspondent Terry Moran tweeted Saturday that Princeton had already lost 11 prospective Ph.D. candidates in computer science.

A friend who works at a top research university's computer science department says they've lost 11 prospective Ph.D candidates to the ban. — Terry Moran (@TerryMoran) January 28, 2017

Princeton’s dean issued a warning on Saturday morning that “strongly advised” students and scholars who may be affected to avoid traveling outside the U.S. for the time being. Immigration attorneys and advocacy groups throughout the country have been giving similar counsel.