Washington DC - A federal judge on Thursday put a temporary block on President Donald Trump's attempt to restrict birthright citizenship.

The ruling imposes a 14-day halt on the enforcement of one of the most controversial executive orders Trump signed hours after being sworn into office for a second term.

It comes after a flurry of lawsuits were filed by a total of 22 states, two cities, and numerous civil rights groups.

"This is a blatantly unconstitutional order," senior US District Judge John Coughenour was reported as saying during the hearing in Washington state.

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"I've been on the bench for over four decades, I can't remember another case where the question presented is as clear as this one is," said Coughenour, who was appointed to the bench by a Republican president, Ronald Reagan.

Birthright citizenship is fundamental to America's national identity, with the 14th Amendment to the US Constitution decreeing that anyone born on US soil is a citizen.

It says, in part: "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside."

Trump's order was premised on the idea that anyone in the US undocumented, or on a visa, was not "subject to the jurisdiction" of the country, and therefore excluded from this category.