Battling with one branch of government and opening a new confrontation with another, US President Donald Trump has declared a national emergency to fulfill his pledge to construct a wall along the US-Mexico border.

Bypassing Congress, which approved far less money for his proposed wall than he had sought, Trump signed the order on Friday (US time) said he will use executive action to siphon billions of dollars from federal military construction and counterdrug efforts for the wall, aides said.

Trump made the announcement at a White House press conference, where he claimed he needs to use emergency powers to protect the nation from drug dealers and illegal immigrants, referring to an "an invasion of our country" from Mexico.

Trump said many former presidents have declared national emergencies. But the presidents he cited did not use emergency powers to pay for projects that Congress wouldn't support.

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Emergency declarations by former presidents Barack Obama, George W. Bush and Bill Clinton were overwhelmingly for the purpose of addressing crises that emerged abroad.

Opposition to the emergency declaration has been swift. Rising Democrat star Alexandria Ocasio-Cortez announcing she is planning to introduce a bill to Congress that would block the move.

Bloomberg reported that Ocasio-Cortez, also known by her initials AOC, will join with Democrat Joaquin Castro to stop Trump doing something many experts have claimed is illegal.

"If President Trump declares a national emergency to fund his border wall, I'm prepared to introduce a resolution to terminate the President's emergency declaration," Castro said before Trump had signed the document.

Trump greets guests on the tarmac as he arrives at West Palm Beach International Airport in West Palm Beach. Photo / AP
Trump greets guests on the tarmac as he arrives at West Palm Beach International Airport in West Palm Beach. Photo / AP

The National Emergencies Act gives Congress the authority to do so by enacting a joint resolution, Castro said.

Democrat leaders denounced Mr Trump's declaration of a national emergency along the US-Mexico border as an unlawful power grab, saying it would do "great violence" to the constitution.

"The President's unlawful declaration over a crisis that does not exist does great violence to our Constitution and makes America less safe, stealing from urgently needed defence funds for the security of our military and our nation," House Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer said in a joint statement.

President Donald Trump and first lady Melania Trump walk to Marine One on the South Lawn of the White House in Washington. Photo / AP
President Donald Trump and first lady Melania Trump walk to Marine One on the South Lawn of the White House in Washington. Photo / AP
Democrat Joaquin Castro. Photo / Getty Images
Democrat Joaquin Castro. Photo / Getty Images

"This is plainly a power grab by a disappointed President, who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process."

Congress has given Mr Trump about $1.4 billion for border barriers, well below the $5.7 billion the president has insisted he needed to build a wall.

The move is already drawing bipartisan criticism on Capitol Hill and is expected to face rounds of legal challenges.

Mr Trump said he expected legal challenges to his decision but says he'll be vindicated.