(NEW YORK) -- A federal judge has blocked the removal of a Palestinian activist from the United States while weighing a petition challenging his arrest, court documents show.
Mahmoud Khalil was arrested by Immigration and Customs Enforcement at Columbia University over the weekend, despite having a green card, his attorney told ABC News, sparking an outcry from civil rights groups.
ICE agents arrested Khalil -- a leader in the Manhattan university's encampment movement -- on Saturday night, claiming that his student visa had been revoked, his attorney, Amy Greer, said in a statement to ABC News.
However, Khalil is in the United States on a green card and not on a student visa, Greer said Sunday. Despite informing agents about his legal status, ICE detained him, she said.
President Donald Trump claimed Khalil was a "Radical Foreign Pro-Hamas Student" and said this is the "first arrest of many to come" in a post on his Truth Social platform on Monday.
"We will find, apprehend, and deport these terrorist sympathizers from our country -- never to return again," he added.
Department of Homeland Security spokesperson Tricia McLaughlin said Khalil is a former Columbia University graduate student and was arrested by ICE "in support of President Trump's executive orders prohibiting anti-Semitism."
"Khalil led activities aligned to Hamas, a designated terrorist organization," McLaughlin said in a statement Sunday night. "ICE and the Department of State are committed to enforcing President Trump's executive orders and to protecting U.S. national security."
Secretary of State Marco Rubio also shared an article about Khalil on Sunday night and posted on X, "We will be revoking the visas and/or green cards of Hamas supporters in America so they can be deported."
Baher Azmy, one of the lawyers representing Khalil, called his client's alleged alignment with Hamas "false and preposterous."
"So setting aside the false and preposterous premise that advocating on behalf of Palestinian human rights and to plead with public officials to stop an ongoing genocide constitutes alignment with Hamas, his speech is absolutely protected by the Constitution, and it should be chilling to everyone that the United States government could punish or try to deport someone because they disapprove of the speech they're engaged in," Azmy told ABC News on Monday.
Greer said she is challenging Khalil's arrest, which occurred at his university residence.
"Overnight we filed a habeas corpus petition on Mahmoud's behalf challenging the validity of his arrest and detention," she said Sunday.
At one point during a phone call with agents, they hung up on Greer, a representative of her law firm told ABC News.
Greer said they initially did not know Khalil's whereabouts following his arrest. His wife, who is eight months pregnant and a U.S. citizen, was unable to find him at an ICE facility in Elizabeth, New Jersey, where he was purportedly transferred, Greer said -- adding that he might have been transferred to Louisiana.
An attorney for Khalil confirmed to ABC News on Monday that he's currently being held at a detention facility in Jena, Louisiana.
"ICE's arrest and detention of Mahmoud follows the U.S. government's open repression of student activism and political speech, specifically targeting students at Columbia University for criticism of Israel's assault on Gaza," the lawyer said. "The U.S. government has made clear that they will use immigration enforcement as a tool to suppress that speech."
The Council on American-Islamic Relations, the nation's largest Muslim civil rights and advocacy group, called for Khalil's release.
"The Department of Homeland Security's lawless decision to arrest him solely because of his peaceful anti-genocide activism represents a blatant attack on the First Amendment's guarantee of free speech, immigration laws, and the very humanity of Palestinians," the group said in a statement on Monday. "We and other civil rights groups are in communication with Mahmoud's legal counsel. This fight is just starting."
Several hundred protesters massed Monday in lower Manhattan to demand his release, carrying signs that said "No political prisoners in the United States" and "Release Mahmoud Khalil."
"He was arrested for speaking out against genocide," said Marian Osman, one of the organizers. "We have a right to free speech. Political speech is protected speech in this country."
After Trump posted that Khalil would not be the last pro-Palestinian demonstrator arrested, Osman pledged protesters would not be intimidated.
"We are just getting started," she said. "I don't think protesters should be scared. I think protesters should be upset and angry."
The arrest occurred just days after Trump took to social media threatening to defund universities that allowed "illegal protests" and claiming "agitators" will be sent back to their home countries.
"All Federal Funding will STOP for any College, School, or University that allows illegal protests. Agitators will be imprisoned/or permanently sent back to the country from which they came. American students will be permanently expelled or, depending on on [sic] the crime, arrested. NO MASKS! Thank you for your attention to this matter," Trump posted on Truth Social on March 4.
Columbia released the following statement on Sunday: "There have been reports of ICE around campus. Columbia has and will continue to follow the law. Consistent with our longstanding practice and the practice of cities and institutions throughout the country, law enforcement must have a judicial warrant to enter non-public University areas, including University buildings."
"Columbia is committed to complying with all legal obligations and supporting our student body and campus community," the statement continued.
ABC News' Jack Feeley contributed to this report.
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