US President Donald Trump signed an executive order on Friday, declaring a national emergency to bypass Congress and appropriate $8 billion he needs to build his long-desired wall along the US-Mexico border.

Minutes after Trump’s long, rambling speech from the White House Rose Garden explaining why he must build the wall, his press secretary Sarah Sanders released a picture, with the caption: “President Trump signs the Declaration for a National Emergency to address the national security and humanitarian crisis at the Southern Border.”

In the speech, President Trump said he was ready to sign an executive order, declaring a national emergency, a move likely to lead to a fierce legal battle between the supporters and opponents of the wall. The Democratic Party, which now has a commanding majority in the House of Representatives, is likely to lead the battle, which may be fought inside the US Congress as well as in the courts.

“I am going to be signing a national emergency,” Trump said after a long introduction to the need to build the fence that touched on trade, China and the caravans of immigrants that he said were heading towards the southern border.

“We’re talking about an invasion of our country with drugs, with human traffickers,” he said while explaining why America needs the wall.

The president said he will also sign a bill approved by Congress on Thursday night that funds the government and prevents a new shutdown set to begin on Saturday.

The bill, however, does not provide the $5.7 billion that Trump said he needed to build the wall.

The American Civil Liberties Union (ACLU), one of the largest civil rights groups in the United States, announced that it will challenge the emergency in the courts.

“President Trump’s hankering for a wall at the southern border cannot be justified by calling a national emergency. This would be a clear abuse of presidential power — one that sidesteps the role of Congress in the appropriation of funds,” ACLU said in a statement issued after the Trump speech.

“Shame on any member of Congress who doesn’t clearly and vigorously speak out on this illegitimate invocation of emergency authorities.”

The majority leader in the US Senate Mitch McConnell said on Thursday evening that he would support the president’s move to assume emergency powers for building the wall without congressional approval.

The emergency allows the chief executive to tap into funds already allocated by Congress for other purposes to bridge the gap between the funding for his border wall. Congress has authorised the executive to use $1.38 billion to build the wall but the administration says it needs $5.7bn for this purpose.

Minutes before Senator McConnell’s announcement, the Senate voted 83-16 to advance the spending package in anticipation of final passage on Thursday night by the House of Representatives. Democrats control the House.

Despite the emergency dispute, it seems that the threats of another government shutdown are not as imminent as it seemed on Thursday. The funding bill that Trump is signing ends a two-month war of attrition between the president and Congress that closed much of the federal government for 35 days. But the declaration of emergency "could instigate a new constitutional clash over who controls the federal purse," the New York Times noted.

House Speaker Nancy Pelosi, a California Democrat, said they were "reviewing options" for responding to Trump’s anticipated declaration and could also take legal actions to prevent him from transferring money from other funds.