Donald Trump declared “our country is full” as he inspected a refurbished section of steel fencing along the Mexican border.
The president has insisted the US immigration system is overburdened and illegal crossings must be stopped.
Claiming there has been a sharp rise in illegal crossings, Mr Trump said: “There is indeed an emergency on our southern border.
“It’s a colossal surge and it’s overwhelming our immigration system, and we can’t let that happen… we can’t take you anymore.
“We can’t take you. Our country is full.”
Mr Trump was speaking at a briefing on immigration and border security in Calexico, California.
He inspected a new two-mile see-through steel-slat barrier that has replaced an older barrier – a long-planned refurbishment divorced from his demands for a wall along the US-Mexico border.
As the president showed off the renovated section of the barrier to reporters, a balloon depicting Mr Trump as a baby floated further down the border.
Mr Trump’s remarks were criticised by rival Democrat politicians. One of them, governor Gavin Newsom, said: “Since our founding, this country has been a place of refuge – a safe haven for people fleeing tyranny, oppression and violence.
“His words show a total disregard of the Constitution, our justice system, and what it means to be an American.”
The president was making a renewed push for border security as a central campaign issue for his 2020 re-election.
Mixing fact with fiction, Mr Trump also complained about the Flores legal settlement which governs the treatment of migrant children and families – and blamed “Judge Flores, whoever you may be”.
In actuality, Flores was an unaccompanied 15-year-old girl from El Salvador.
The president also downplayed claims of people seeking asylum at the border, declaring without evidence that many are gang members.
Mr Trump also compared some of their efforts to find safety in the US with special counsel Robert Mueller’s investigation about Russian interference in the 2016 elections.
Mr Trump said: “It’s a scam, it’s a hoax.
“I know about hoaxes. I just went through a hoax.”
California and 19 other states are suing Mr Trump over his emergency declaration to build a border wall.
The states requested a court order to stop money from being diverted to fund the project just as Air Force One touched down in California on Friday.
Mr Trump, who has ratcheted up his hard-line immigration rhetoric in recent weeks, has insisted his move was necessary.
Democrats filed a lawsuit preventing Mr Trump from spending more money than Congress has approved to erect barriers along the southwestern border.
Congress approved just under $1.4bn (£1.07bn) for work on border barricades.
Mr Trump has asserted he can use his powers to transfer an additional $6.7bn (£5.4bn) to wall construction.
The president, who earlier in the week threatened to shut down the border over the high number of migrants trying to enter the US, appeared to walk back his comments on Thursday.
On Friday, Mr Trump said this was because Mexico had gotten tougher in stopping an influx of immigrants from moving north.
As he left the White House, the president added: “Mexico has been absolutely terrific for the last four days.
“I never changed my mind at all. I may shut it down at some point.”
Mr Trump has been increasingly exasperated at his inability to halt the swelling number of migrants entering the US, including thousands who have been released after arriving because border officials have no space for them.
Arrests along the southern border have skyrocketed in recent months, and border agents were on track to make 100,000 arrests or denials of entry in March – a 12-year high.
More than half of those are families with children, who require extra care.
Source: SKY NEWS