WASHINGTON -- Donald Trump on Friday temporarily banned citizens of seven Muslim nations from entering the United States and indefinitely blocked all Syrians seeking refuge here, closing America's gates to millions and forcing US law enforcement to immediately detain or turn away hundreds of visa holders and asylum seekers at its borders.Proclaiming nationals of Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen as threats to national security– roughly 134 million people– the president said his blanket suspension on visa issuances would hold until the Department of Homeland Security institutionalizes new vetting procedures. Those guidelines will include a religious test of some kind, said Trump, who promised in an interview that he would grant priority to Christians seeking asylum over persecuted Muslims.Trump has long pledged to take this kind of action, making it a prominent feature of his campaign for the White House. But people who work with Muslim immigrants and refugees were scrambling on Friday night to determine the scope of the order."The United States cannot, and should not, admit those who do not support the Constitution, or those who would place violent ideologies over American law," the executive order reads. Trump explained his decision at a signing ceremony at the Pentagon: "I'm establishing new vetting measures to keep radical Islamic terrorists out of the United States of America. We don't want them here."Trump signed the order, titled "Protecting the Nation from Foreign Terrorist Entry Into the United States," on International Holocaust Remembrance Day– a decision that riled American Jewish groups."The terrorist threat attributed to refugees is a cruel and distracting fiction, especially when viewed against the actual incidence of mass violence committed with chilling frequency– in schools, churches, shopping malls and other venues– against Americans by Americans," the American Jewish Committee said in a strong statement. "Blanket suspensions of visas and refugee admission would suggest guilt by association– targeted primarily at Muslims fleeing violence and oppression."Several Jewish human rights organizations, including T'ruah, Hebrew Immigrant Aid Society (HIAS) and Bend the Arc, mobilized on Friday to fight Trump's action in courts of law and public opinion. So too did the Anti-Defamation League, which vowed a "relentless fight" against an order that it characterized as a fundamental challenge to Jewish values."History will look back on this order as a sad moment in American history– the time when the president turned his back on people fleeing for their lives," reads the ADL statement. "This will effectively shut America’s doors to the most vulnerable people in the world who seek refuge from unspeakable pain and suffering.""Our history and heritage compel us to take a stand," it adds.Over 1,700 American rabbis signed a letter in support of the US refugee resettlement program, and nearly 600 Soviet Jews who emigrated to the US signed a petition in opposition to Trump's decision "to close America’s doors to vulnerable refugees desperately seeking our protection.""Ending all admission of refugees? A religious test for those admitted to the country? Legal immigrants denied reentry? Ugly all around," wrote Dan Shapiro, former ambassador to Israel under US President Barack Obama, describing himself as "sickened" by the move.While Trump says this measure is meant to protect the homeland, the White House offered no explanation as to why he believes existing programs are not working: There have been no attacks in the United States perpetrated by nationals from the seven nations listed in Friday's order. Nevertheless, all seven have, at one point or another, been included in the State Department's list of state sponsors of terrorism.All nineteen hijackers behind the September 11th attacks hailed from the nations of Saudi Arabia, the United Arab Emirates, Egypt and Lebanon– none of which are included on Trump's list. Pakistan and Indonesia, the world's two largest Muslim-majority nations, are also not included. But Trump said in an ABC News interview that nationals from these countries would face "extreme vetting" at America's borders.Legal permanent residents from the seven listed nations– people with "green cards" allowing them to live and work in the United States– were being advised to consult immigration lawyers before traveling outside the country, or trying to return, said Muslim Advocates, a civil rights group in Washington. Homeland Security say that green card holders are included in the ban. Confusion and anxiety gripped America's airports in the hours after Trump signed the order, which went into immediate effect, as foreign nationals who had successfully obtained US visas were turned away. Dozens who were airborne as Trump took action were subsequently detained at the gates. And several of those are filing lawsuits against the government in response.Throughout his presidential campaign, Trump warned that Islamic State would send a "Trojan horse" into the homeland through America's refugee resettlement program. Tapping into the nation's fears, he floated "a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on."He walked that back somewhat by tailoring his ban to specific nations that he characterizes as "terror prone." Nations on his initial list are either formal Islamic states or recognize Islam as the national religion.Trump said in an interview with the Christian Broadcasting Network that the exception would help Syrian Christians fleeing the civil war there.That proposal has opened him up to legal challenges: The Council on American-Islamic Relations said the order targets Muslims because of their faith, contravening the US Constitutional right to freedom of religion, and vowed to file a suit in court on Monday. "President Trump has cloaked what is a discriminatory ban against nationals of Muslim countries under the banner of national security," said Greg Chen of the American Immigration Lawyers Association.Legal experts were divided on whether this order would be constitutional. "If they are thinking about an exception for Christians, in almost any other legal context discriminating in favor of one religion and against another religion could violate the constitution," said Stephen Legomsky, a former chief counsel at US Citizenship and Immigration Services in the Obama administration.But Peter Spiro, a professor at Temple University Beasley School of Law, said Trump's action would likely be constitutional because the president and Congress are allowed considerable deference when it comes to asylum decisions. "It's a completely plausible prioritization, to the extent this group is actually being persecuted," Spiro said.Republican Speaker Paul Ryan, who had panned Trump's original campaign pledge to ban Muslims from entering the United States, expressed tepid support for the move on Friday. "We are a compassionate nation, and I support the refugee resettlement program, but it's time to reevaluate and strengthen the visa vetting process," Ryan said.The order is affecting some Iraqi nationals who have worked for US since its invasion of Iraq in 2003. One Iraqi interpreter, Hameed Khalid Darweesh, who worked for the Army's 101st Airborne Division, was detained for fourteen hours at New York's Kennedy Airport on Friday despite holding a special immigrant visa.Darweesh had been targeted twice in Iraq for working for the American military, according to his lawyers, who are taking action against the government.Reuters contributed to this report.