The Republican president's move to circumvent Congress represented an escalation in his efforts to make good on a 2016 presidential campaign pledge to build a wall to halt the flow into the country of illegal immigrants, who Trump says bring crime and drugs.
Hours after Trump's announcement, the Democratic-controlled House of Representatives' Judiciary Committee said it had launched an investigation into the emergency declaration.
In a letter to Trump, committee Democrats asked him to make available for a hearing White House and Justice Department officials involved in the action. They also requested legal documents on the decision that led to the declaration, setting a deadline of next Friday.
"We believe your declaration of an emergency shows a reckless disregard for the separation of powers and your own responsibilities under our constitutional system," said the letter signed by Chairman Jerrold Nadler and other top Democrats on the panel.
Trump on Friday also signed a bipartisan government spending bill that would prevent another partial government shutdown by funding several agencies that otherwise would have closed on Saturday.