President Trump tweeted Monday night that Immigration and Customs Enforcement will begin removing "millions" of undocumented immigrants. Mr. Trump insisted Tuesday the immigration roundup would begin next week, targeting people who have open deportation orders.

"When people come into our country and they come in illegally, they have to go out," he said.

The president has been pushing immigration officials to take a harder line and recently appointed Mark Morgan to lead Immigration and Customs Enforcement (ICE).

The news shocked immigration advocates and public officials like Oakland Mayor Libby Schaaf, who famously warned residents of a pending ICE raid last year. "This president continues to fear monger, he continues to spread racist lies," she said.

While most crossing illegally into the United States recently won't be affected, those that help them say the threat of a tougher tone is meant to scare and deter. Last month alone, almost 145,000 migrants crossed into the U.S. from Mexico and most asked for political asylum.

"My guess is that for the next 48 hours or so, you will see a lot of immigrant kids who are not in school, parents are not going to show up for work," said Theresa Cardinal Brown, a former Homeland Security official.

In Phoenix, many recent migrants were dropped at bus stations to make their way to families and await court hearings on asylum claims. Connie Phillips helped hundreds. She said she doesn't think the president's tough talk will stem the flow of immigrants.

"It's not what's driving it. What's driving it are the issues that they're experiencing in their own countries," Phillips said.