The school in Cambridge, Massachusetts quickly slammed the move as "unlawful" and said it would hurt both the campus and the country, while one student said the community was "panicking."

Trump is furious at Harvard -- which has produced 162 Nobel prize winners -- for rejecting his demand that it submit to oversight on admissions and hiring over his claims that it is a hotbed of anti-Semitism and "woke" liberal ideology.

The loss of such a large proportion of the student body could prove to be a huge financial blow to Harvard, which charges tens of thousands of dollars a year in tuition.

"Effective immediately, Harvard University's Student and Exchange Visitor (SEVIS) Program certification is revoked," Homeland Security Secretary Kristi Noem wrote in a letter to the Ivy League institution, referring to the main system by which foreign students are permitted to study in the United States.

Harvard, which has sued the government over a separate raft of punitive measures, quickly fired back, calling the move "unlawful."

"We are fully committed to maintaining Harvard's ability to host our international students and scholars," it said in a statement, adding that it was working to offer students guidance and support. 

"This retaliatory action threatens serious harm to the Harvard community and our country, and undermines Harvard's academic and research mission."

Karl Molden, an international student from Austria, said he had applied to study at Oxford in Britain because he feared such measures.

"It's scary and it's saddening," the 21-year-old government and classics student told AFP.

"I love Harvard, and getting into the school has been the greatest privilege of my life.

"It's definitely going to change the perception of... students who (might) consider studying there -- the US is getting less of an attractive spot for higher education."

Leaders of the Harvard chapter of the American Association of University Professors called the move "the latest in a string of nakedly authoritarian and retaliatory moves against America's oldest institution of higher education."

"The Trump administration is unlawfully seeking to destroy higher education in the United States. It now demands that we sacrifice our international students in the process. Universities cannot acquiesce to such extortion," it said.

Last month, Trump threatened to stop Harvard from enrolling foreign students if it did not agree to government demands that would put the private institution under outside political supervision.

"As I explained to you in my April letter, it is a privilege to enroll foreign students," Noem wrote. 

"All universities must comply with Department of Homeland Security requirements, including reporting requirements under the Student and Exchange Visitor Program regulations, to maintain this privilege," she said.

"As a result of your refusal to comply with multiple requests to provide the Department of Homeland Security pertinent information while perpetuating an unsafe campus environment that is hostile to Jewish students, promotes pro-Hamas sympathies, and employs racist 'diversity, equity, and inclusion' policies, you have lost this privilege."

More than 27 percent of Harvard's enrollment was made up of foreign students in the 2024-25 academic year, according to university data.

Fourth-year US student Alice Goyer told AFP "no one knows" what the development would mean for international students already enrolled. 

"We just got the news, so I've been getting texts from a lot of international friends, and I think everyone's just -- no one knows," she said.

"Everyone's panicking a bit."

On whether students would willingly transfer to other institutions, as suggested by Noem in the letter, Goyer said, "I doubt people would do that."

"I would hope maybe there's going to be a legal battle that'll take place."