Donald Trump signed the first veto of his presidency Friday, overriding a congressional resolution to secure emergency funds to build his much-vaunted wall on the US-Mexico border.

Trump declared in the Oval Office that he was "proud" to sign the veto.

"Congress has the freedom to pass this resolution and I have the duty to veto it," he said.

Trump suffered an embarrassing defeat on Thursday when senators, including fellow Republicans, voted to terminate his declaration of an emergency on the Mexican border.

Surrounded by law enforcement officials, senior aides and people who have lost loved ones to cross-border crime, Trump said the veto was to "defend the safety of all Americans."

"The mass incursion of illegal aliens... has to end," he said. "People hate the word 'invasion' but that’s what it is... Our immigration system is stretched beyond the breaking point."

Trump's emergency declaration allows him to secure funding for construction of border walls after he failed to get authorization from Congress. 

Opponents, who accuse Trump of executive overreach and overhyping the problem on the border, could now use court challenges to halt the emergency measure.