US President Donald Trump has defended his move to impose a national emergency in the country, saying that "we have an invasion of drugs, gangs, people, and it's unacceptable".

He also confirmed that he will use emergency powers to build a wall on the US border with Mexico, saying "walls work", the BBC reported on Saturday. 

"We're going to confront the national security crisis on our southern border," he added.

The President's plan was announced after the Congress refused to pay for a wall. California Governor Gavin Newsom, meanwhile, announced plans to sue the Trump administration after the President's move for national emergency to access funds for building the wall on the Mexican border was made public. 

Trump had already anticipated 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.

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 preserve and three Texas landowners who have been told the wall may be constructed on their properties.

The emergency declaration will give Trump access to billions of dollars for his project. The President said the emergency would allow him to get almost $8 billion for the wall.

The money is expected to be diverted from military construction projects and efforts to fight the drugs trade.

This is still considerably short of the estimated $23 billion cost of the wall along almost 3,200 kilometres of the US-Mexico border.

Building the wall was a key pledge of Trump's campaign in 2016. The Democrats have described the move as a "gross abuse of power" ahead of the 2020 election.