Rights groups are amassing in opposition to President Donald Trump's executive order banning immigrants from Muslim countries, which he signed Friday after a meeting with British Prime Minister Theresa May.

The Council on American-Islamic Relations (CAIR) said it would file a federal lawsuit challenging the constitutionality of the memorandum, which bans immigrants from Syria indefinitely and halts arrivals from Iran, Iraq, Yemen, Sudan, Libya, and Somalia for 90 days.

Refugees who were in the air on the way to the U.S. on Friday when Trump signed his order were detained when they landed, the New York Times reported Saturday.

Meanwhile, the United Nations refugee agency (UNHCR) and the International Organization for Migration (IOM) called on Trump to rescind the order, cautioning that the U.S. resettlement program was critical, particularly in a time of global turmoil.

"The needs of refugees and migrants worldwide have never been greater and the U.S. resettlement program is one of the most important in the world," the organizations said in a joint statement.

"Tears are running down the cheeks of the Statue of Liberty tonight as a grand tradition of America, welcoming immigrants, that has existed since America was founded, has been stomped upon," said Senate Minority Leader Chuck Schumer (D-N.Y.).

"Taking in immigrants and refugees is not only humanitarian but has also boosted our economy and created jobs decade after decade. This is one of the most backward and nasty executive orders that the president has issued," he said.

The plan was announced earlier this week amid a flurry of executive orders cracking down on immigration.

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The order, officially titled the Protection of the Nation from Foreign Terrorist Entry into the United States—which Trump termed "extreme vetting" designed to "keep terrorists out"—was more severe than expected and effectively blocks Muslims traveling to the U.S. from wide swaths of the Middle East and North Africa.

That's despite the fact that the U.S. already has the most rigorous screening process in the world, as the Washington Post points out. The memorandum also prioritizes admission for Christians and caps the total number of refugees entering the country in 2017 at 50,000, less than half what it was the previous year.

Moustafa Bayoumi, a Brooklyn College professor, wrote in an op-ed for the Guardian that the order means Trump is "now officially gunning for Muslims."

"The order is nothing short of a Muslim ban by another name. It is cruel and callous, espouses positions contrary to the professed values of the United States, and will certainly produce more problems than it purports to solve," Bayoumi wrote. "In other words, it's exactly like Donald Trump."

Anthony D. Romero, executive director of the American Civil Liberties Union (ACLU), likewise warned Friday, "'Extreme vetting' is just a euphemism for discriminating against Muslims. Identifying specific countries with Muslim majorities and carving out exceptions for minority religions flies in the face of the constitutional principle that bans the government from either favoring or discriminating against particular religions. Any effort to discriminate against Muslims and favor other religions runs afoul of the First Amendment."

Announcing the lawsuit Friday, CAIR national litigation director Lena F. Masri said in a statement, "There is no evidence that refugees—the most thoroughly vetted of all people entering our nation—are a threat to national security. This is an order that is based on bigotry, not reality."

Gadeir Abbas, a co-counsel in the lawsuit, added, "The courts must do what President Trump will not—ensure that our government refrains from segregating people based on their faith."