Former US president Barack Obama decried his successor's decision to end an amnesty for 800,000 people brought to America illegally as children, describing it as "wrong," "self-defeating" and "cruel."
In a rare re-entry onto the political stage, Obama used a Facebook post to slam Trump's decision and call on Congress to step in.
"To target these young people is wrong -- because they have done nothing wrong," he wrote.
"It is self-defeating -- because they want to start new businesses, staff our labs, serve in our military, and otherwise contribute to the country we love. And it is cruel."
"What if our kid's science teacher, or our friendly neighbor turns out to be a Dreamer? Where are we supposed to send her? To a country she doesn't know or remember, with a language she may not even speak?"
Obama, who is currently finishing his memoirs, had vowed before leaving office that he would speak up if Trump moved to unravel the amnesty introduced in 2012 -- during the ex-president's first term.
Trump has argued that the policy was legally flawed, and that he must put native-born Americans first.
Obama rubbished that suggestion.
"Let's be clear: the action taken today isn't required legally. It's a political decision, and a moral question."
"Whatever concerns or complaints Americans may have about immigration in general, we shouldn't threaten the future of this group of young people who are here through no fault of their own, who pose no threat, who are not taking away anything from the rest of us."
"It's up to Members of Congress to protect these young people and our future."