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."
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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.
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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 FridaRead More – Source
