Attorney General Jeff Sessions made the announcement at a Tuesday press briefing that President Barack Obama's DACA program would be dismantled, calling the program "an unconstitutional exercise of authority."
New applicants will be halted for DACA, which has provided nearly 800,000 young immigrants a reprieve from deportation and the ability to work legally in the U.S. in the form of two-year, renewable work permits.
The administration is giving Congress six months to come up with a legislative fix - "should it choose to," Sessions said - before the government stops renewing permits for people already covered by the program.
MORE: What you need to know about DACA
According to Department of Homeland Security officials, people with permits whose renewals are set to expire between now and March 5, 2018, will be able to re-apply - so long as their applications are submitted by Oct. 5, 2017, one month from Tuesday. No permits will be revoked before their existing expiration dates, and applications already in the pipeline will be processed, they said.
President Donald Trump, in a statement, said the change would be "a gradual process, not a sudden phase out."
VIDEO: DACA supporters gather outside federal building
"Thus, in effect, I am not going to just cut DACA off, but rather provide a window of opportunity for Congress to finally act," he said. He said he did not favor punishing children for the actions of their parents. At the same time, though, "we must also recognize that we are a nation of opportunity because we are a nation of law" and "young Americans have dreams, too."
Those at the downtown rally called it "a shameful day for America."
"We are as American as anybody else," one Dreamer said.
MORE: Obama releases statement on DACA
"We must fight for a permanent solution, a clean permanent solution like the DREAM Act," said Melody Klingenfuss with the Coalition for Humane Immigrant Rights of Los Angeles (CHIRLA). "We need both Democratic and Republican support for a clean solution for ourselves and for our immigrant families."
Angelica Salas, also with CHIRLA, addressed the president directly.
"What you did is immoral. Your decision is cowardly. It's irresponsible. What you did today was destroy hundreds of thousands of lives, young lives. You picked on the youngest Americans," Salas said.
The questions on the minds of many at the rally were: What happens in six months? In terms of deportations, how aggressive will authorities be?
California has more DACA recipients than any other state - more than 200,000 since 2012. In Southern California alone, there are a little more than 100,000 Dreamers, according to numbers provided by Los Angeles County.
The chancellor of the Los Angeles County Community College District said about 11,000 DACA students attend campuses all over the county. He urges them not to do anything that could negatively affect their status in the county over the next six months.
About 150 demonstrators gathered at the federal building on Tuesday and marched to the L.A. County Board of Supervisors on Temple Street, where Mayor Eric Garcetti and other officials spoke out against the Trump administration's decision to rescind DACA.
"So we say clearly to Washington, get your act together and do what's right for America and pass the DREAM Act once and for all - right now," Garcetti said at the event.
A rally was also underway on Olvera Street, where many DACA beneficiaries will be protesting. Local immigrant advocates mobilized to help DACA recipients. Among those willing to help are lawyers at Loyola Law School's Immigrant Justice Clinic.
Those lawyers said their clinic will be assisting DACA recipients whose work permits will soon be expiring.
"It is very important to assess all of your legal options. You may have DACA because you realize you're eligible, but you may be eligible for something that could lead to a green card," said Emily Robinson, with the clinic. "At this point, when we don't know who is being prioritized and we are unsure of enforcement actions, it's very important to take the time to make sure you understand the entire legal landscape of your status."
The staff at the clinic will be providing legal assistance to immigrants at the Mexican Consulate. The consulate will also offer extended hours until the DACA deadline.