Donald Trump has declared a national emergency in a bid to fund his promised wall at the US-Mexico border without congressional approval, an action Democrats vowed to challenge as a violation of the US Constitution.

The Republican president’s move to circumvent Congress represented a new approach to making good on a 2016 presidential campaign pledge to halt the flow of undocumented immigrants into the country, whom the president says bring crime and drugs.

He also later signed a bipartisan government spending bill Congress approved on Thursday that would prevent another partial government shutdown by funding several agencies that otherwise would have closed on Saturday.

Mr Trump made no direct mention in rambling Rose Garden comments of the funding bill. It represents a legislative defeat for him since it contains no money for his proposed wall - the focus of weeks of conflict between him and Democrats in Congress.

He had demanded that Congress provide him with billions in wall funding as part of legislation to fund the agencies. That triggered a historic, 35-day December-January government shutdown that hurt the US economy and his opinion poll numbers.

By reorienting his quest for wall funding toward a legally uncertain strategy based on declaring a national emergency, Mr Trump risks plunging into a lengthy legislative and legal battle with Democrats and dividing his fellow Republicans.

At least 15 Democrats in the Republican-controlled Senate introduced legislation on Thursday to prevent Mr Trump from invoking emergency powers to transfer funds to his wall from accounts Congress has already committed to other projects.

Nancy Pelosi, the Democratic speaker of the House of Representatives, and top Senate Democrat Chuck Schumer swiftly responded to Trump’s declaration.

“The president’s actions clearly violate the Congress’s exclusive power of the purse, which our Founders enshrined in the Constitution,” they said in a statement. “The Congress will defend our constitutional authorities in the Congress, in the courts, and in the public, using every remedy available.”

Reuters contributed to this report. Check out The Independent's live coverage of the president's national emergency declaration below: