The president met with mayors, sheriffs and other leaders from California at the White House on Wednesday, all of whom back his hard-line stance on immigration.
When presented with a complaint about the criminal gang MS-13, Mr Trump said: “We have people coming into the country, or trying to come in, and we’re stopping a lot of them.
“You wouldn’t believe how bad these people are. These aren’t people. These are animals.”
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Democrats have been quick to criticise the president for his rhetoric, which Republicans see as a potentially galvanising issue for his hardcore supporters during the midterm elections later this year.
Mr Trump has been a vocal critic of California’s so-called sanctuary immigration policies, with laws introduced last year barring police from asking people about their immigration status or helping federal agents with immigration enforcement.
The White House has sued to reverse the legislation, describing it as “unconstitutional and dangerous”, with Mr Trump claiming that it allowed “the release of illegal immigrant criminals, drug dealers, gang members and violent predators”.
“We have the dumbest laws on immigration in the world,” he said during his round-table with Californian leaders.
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He accused Democratic California Governor Jerry Brown of providing “safe harbour to some of the most vicious and violent offenders on earth”, but Mr Brown has said that the president is “lying” about immigration and his policies.
He added: “Flying in a dozen Republican politicians to flatter him and praise his reckless policies changes nothing. We, the citizens of the fifth largest economy in the world, are not impressed.”
Other Democrats have also waded in to the spat to accuse Mr Trump of “dehumanising” people coming into the country, including Senator Chuck Schumer.
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“When all of our great-great-grandparents came to America, they weren’t animals,” he said on Twitter.
Colorado Congressman Jared Polls added: “Immigrants are human beings. Not animals, not criminals, not drug dealers, not rapists. They are human beings.”