WASHINGTON, United States (AFP) — Donald Trump signed the first veto of his presidency Friday, overriding congressional opposition to secure emergency funds to build more walls on the US-Mexico border.

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

It came after he 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 reaffirming his power to get the funds without Congress 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.

Nancy Pelosi, the Democratic speaker of the House of Representatives and a leader in the fight to prevent Trump’s wall plans, called Trump’s action a “lawless power grab.”

“The president has chosen to continue to defy the Constitution, the Congress and the will of the American people,” she said in a statement.

An attempt to override the veto will be held on March 26, she announced. However, this is very unlikely to pass as a two-thirds majority is required.

Trump has made border security an over-arching domestic issue in his presidency and says it will remain at the centre of the agenda in his 2020 re-election bid.

Although there has been a surge in arrival of families and children at the border, overall apprehensions at the boundary are down substantially from a decade or more ago.

Most Republicans support Trump’s position that the border is out of control. However, there were defections in Thursday’s Senate vote by Republican senators angered at what they see as Trump’s improper seizing of power over the government purse strings — a role reserved for the legislature.