President Trump faces legal challenges to his decision to use emergency powers to build a wall on the US border with Mexico.
California and New York said they would take legal action to challenge his move to bypass Congress and secure funding for the project.
Building the wall was a key pledge of Mr Trump’s campaign.
Democrats said it was a “gross abuse of power” and vowed to contest it “using every remedy available”.
On Friday, Mr Trump signed the emergency declaration along with a spending bill aimed at preventing a repeat of a recent government shutdown.
Declaring an emergency could give him access to billions of dollars. Mr Trump announced the plan after Congress refused funding for the wall.
Within hours, the first legal challenge against the declaration of national emergency was launched.
A liberal advocacy group, Public Citizen, sued on behalf of a nature reserve and three Texas landowners who have been told the wall may be constructed on their properties.
How have Democrats responded?
Governor Gavin Newsom of California dismissed the president’s decision as “political theatre”.
“He’s been embarrassed, and his base needs to be fed,” he told reporters.
“Fortunately, Donald Trump is not the last word. The courts will be the last word,” he added.
New York state’s Democratic attorney general, Letitia James, said the state would not “stand for this abuse of power and will fight back with every legal tool at our disposal.”
The American Civil Liberties Union (ACLU) said it would file a lawsuit in the coming days to curb “this blatantly illegal executive action”.
On Friday the two most senior Democrats – House of Representatives Speaker Nancy Pelosi and Senate Democrat leader Chuck Schumer – said they would challenge the “power grab by a disappointed president” in Congress and in the courts.
Ms Pelosi also seized on a remark by Mr Trump in response to a question from a reporter, in which he said he “didn’t need to do this”.
Analysts suggest that this remark could undermine Mr Trump’s case that the country is facing an emergency.
What did Mr Trump say?
Making the announcement in the White House Rose Garden, the president said the emergency would allow him to get almost $8bn for the wall.
This is still considerably short of the estimated $23bn cost of the wall along almost 2,000 miles (3,200km) of border.
Mr Trump accepted that he would be sued for the move, and predicted that the emergency order would lead to legal action which was likely to end up in the Supreme Court.
“We’re going to confront the national security crisis on our southern border,” he said.
“Everyone knows that walls work.”
Later, Mick Mulvaney, the acting White House chief of staff, told reporters that Mr Trump’s move “creates zero precedent”.
“This is authority given to the president in law already. It’s not as if he just didn’t get what he wanted so he’s waving a magic wand and taking a bunch of money,” he said.