For the last few weeks, many foreign students living in the US have watched as a sequence of events has repeated itself on their social media feeds: plain-clothes agents appearing unannounced and hauling students off in unmarked cars to detention centres.

In a stunning blow to international scholars who had hopes of studying in the United States, the Trump administration has revoked hundreds of student visas in nearly every corner of the country as part of a vast immigration crackdown.

Leaders at many colleges learned the legal status of some of their international students had been terminated when they checked a database managed by Homeland Security. In the past, college officials say, visas typically were revoked after schools updated the government when students fell out of status.