President Donald Trump says he will revisit the decision to end a program that shielded nearly 800,000 young, undocumented immigrants from deportation if Congress doesn't act on the issue.
Hours after administration officials said new applications for the Deferred Action for Childhood Arrivals program, known as DACA, will no longer be accepted, Trump tweeted late Tuesday that "Congress now has 6 months to legalize DACA (something the Obama administration was unable to do). If they can't, I will revisit this issue!"
Action by Congress is not certain. Lawmakers have been unsuccessful for years in their efforts to revise substantially U.S. immigration policies. During Obama's eight years as president, the Senate - controlled by members of his Democratic Party for most of that time — approved major policy changes only to see the legislation fail in the Republican-controlled House of Representatives.
President Trump approved the decision to end DACA but sent Attorney General Jeff Sessions before news cameras Tuesday to announce the controversial policy change.
"DACA is being rescinded," Sessions announced. The action revoked an executive order former President Barack Obama issued five years ago after the U.S. Congress repeatedly failed to agree on an immigration reform bill.
WATCH: Attorney General Jeff Sessions
Sessions argued that Obama's "open-ended circumvention of immigration laws" was in violation of the U.S. constitution and unlikely to survive a legal challenge brought by several Republican-controlled states.
Former President Barack Obama, who has refrained from commenting on most of the policy changes Trump has enacted this year, challenged Sessions' legal argument in a strongly worded statement, saying the decision was "purely political" and that it targeted young people who "have done nothing wrong."
Demonstrators opposed to the administration's decision massed in Washington, Los Angeles, New York, Denver and other cities.
WATCH: 'Dreamers' Vow to Fight to Keep DACA Until the Bitter End
Activist Gustavo Torres told a crowd outside the White House: "This president lied to our community. ... He told us, 'I have a big heart for you dreamers.' He's a liar!"
The future status of the hundreds of thousands of young, foreign-born students and workers is unclear for now, since they are no longer protected from summary deportation by the DACA program. Congress will have six months to act if it wants to continue to allow them to remain in the United States.
The young immigrants, also colloquially known as "dreamers," typically entered the United States as young children. Many trace their heritage to Mexico or Central American countries, but some arrived so young that they have grown up knowing nothing other than American society and customs.
Anyone who joined the "deferred action" program for work and study was required to have and maintain a clean criminal record. DACA did not promise participants citizenship or permanent U.S. residency, instead promising a reprieve from deportation.
DACA Changes Explained
The program was initially intended as a stop-gap measure to protect aspiring young immigrants, while Congress was to come up with a more lasting solution to their problems.
"I have a love for these people," Trump said at the White House late Tuesday, "and hopefully now Congress will be able to help them and do it properly." Earlier he had issued only a written statement stating that federal immigration patrols would not make seeking out DACA recipients for detention and deportation a priority issue.
Victoria Machi and Ramon Taylor contributed to this report.