WASHINGTON, Jan 20 (Reuters) - President Donald Trump on Monday kicked off his sweeping immigration crackdown, declaring illegal immigration at the U.S.-Mexico border a national emergency, designating criminal cartels as terrorist organizations and taking steps to block citizenship for children of immigrants in the U.S. illegally.

The series of executive orders that Trump outlined in his inaugural address, said he would invoke a 1798 wartime law known as the Alien Enemies Act to target foreign gang members in the U.S., a legal authority last used to detain non-citizens of Japanese, German, and Italian descent in internment camps during World War Two.

Shortly after the inauguration, U.S. border authorities said they had shut down outgoing President Joe Biden's CBP One entry program, which had allowed hundreds of thousands of migrants to enter the U.S. legally by scheduling an appointment on an app. Existing appointments were canceled, leaving migrants stunned and unsure of what to do.

Trump, a Republican, recaptured the White House after promising to intensify border security and deport record numbers of migrants. Trump criticized Biden for high levels of illegal immigration during the Democrat's presidency. In June, Biden toughened his policies and Mexico stepped up enforcement, and the number of migrants caught crossing illegally fell dramatically.

Republicans say large-scale deportations are necessary after millions of immigrants crossed illegally during Biden's presidency. There were roughly 11 million immigrants in the U.S. illegally or with a temporary status at the start of 2022, according to a U.S. government estimate, a figure that some analysts now place at 13 million to 14 million.

"As commander-in-chief, I have no higher responsibility than to defend our country from threats and invasions, and that is exactly what I am going to do," Trump said in his address.

Trump's critics and immigrant advocates say mass deportations could disrupt businesses, split families and cost U.S. taxpayers billions of dollars.

The American Civil Liberties Union said in a federal court filing on Monday that Trump's decision to end the CBP One program removed the only avenue to asylum at the U.S.-Mexico border, an opening salvo by the civil rights group to fight Trump's agenda in court.

California and other Democratic-led states whose policies limit cooperation with federal immigration enforcement also could clash with Trump.

Americans have grown less welcoming toward immigrants without legal status since Trump's first presidency, but remain wary of harsh measures such as using detention camps, a Reuters/Ipsos poll in December found.

In several Mexican border cities, migrants saw their appointments on Biden's CBP One app canceled just after Trump took office. Some 280,000 people had been logging into the app daily to secure an appointment as of Jan. 7.

Migrants waiting in Ciudad Juarez scrambled to find short-term rentals, buy bus tickets and call family members back home.

Daynna del Valle, a 40-year-old Venezuelan, spent eight months in Mexico waiting for an appointment that would have arrived on Tuesday. In that time, she worked at a nail salon but earned so little that she barely managed to send any money back to her mother in Colombia, a cancer survivor who needed medical treatment for her blood pressure.

"I'm lost," she said. "I don't know what to do, where to go."

Denia Mendez, a Honduran sitting in the courtyard of a migrant shelter in Piedras Negras - a Mexican city across from Eagle Pass, Texas - opened her email inbox 30 minutes after Trump became president. She stared at an email for several minutes, reading it over and over, before her eyes welled up.

"They canceled my appointment," she said. Several other migrants, who just minutes ago were laughing as they fed potato chips to pigeons, huddled around her phone, their faces suddenly grave.

Mendez's 15-year-old daughter Sofia kept trying to get into the CBP One app.

"They're not going to let you into the app, baby," her mother told her softly.

In his order focused on so-called "birthright citizenship Trump will challenge U.S. citizenship for children born to parents in the U.S. illegally, an incoming Trump official said earlier in the day. The text of the order was not immediately available. The right stems from an amendment to the U.S. Constitution and any move to restrict it will almost certainly trigger legal challenges.

Trump's order dealing with U.S. refugee resettlement will suspend the program for at least four months and will order a review of security to see if travelers from certain nations should be subject to a travel ban, the official said.

Trump said in his address that he would reinstate his first-term "remain in Mexico" program, which forced non-Mexican asylum seekers to wait in Mexico for the outcome of the U.S. cases. Biden ended the program in 2021, saying migrants were stuck waiting in squalid conditions.

"All illegal entry will immediately be halted, and we will begin the process of returning millions and millions of criminal aliens back to the places from which they came," Trump said.

Mexico's presidency, foreign ministry, and economy ministry did not immediately respond to requests for comment on Trump's plans. In a regular press conference on Monday, Mexican President Claudia Sheinbaum called for calm and insisted her government had to see the details of Trump's actions before responding.

Photo from Reuters