A federal judge in Seattle on Thursday blocked President Donald Trump's administration from implementing an executive order curtailing the right to automatic birthright citizenship in the United States, calling it "blatantly unconstitutional."

U.S. District Judge John Coughenour at the urging of four Democratic-led states issued a temporary restraining order preventing the administration from enforcing the order, which the Republican president signed on Monday during his first day on office.

"This is blatantly unconstitutional order," the judge told a lawyer with the U.S. Justice Department defending Trump's order.

The order has already become the subject of five lawsuits by civil rights groups and Democratic attorneys general from 22 states, who call it a flagrant violation of the U.S. Constitution.

"Under this order, babies being born today don't count as U.S. citizens," Washington Assistant Attorney General Lane Polozola told Senior U.S. District Judge John Coughenour at the start of a hearing in Seattle.

Polozola - on behalf of Democratic state attorneys general from Washington state, Arizona, Illinois and Oregon - urged the judge to issue a temporary restraining order to prevent the administration from carrying out this key element of Trump's immigration crackdown.

The challengers argue that Trump's action violates the right enshrined in the citizenship clause of the Constitution's 14th Amendment that provides that anyone born in the United States is a citizen.

Trump in his executive order directed U.S. agencies to refuse to recognize the citizenship of children born in the United States if neither their mother nor father is a U.S. citizen or legal permanent resident.

In a brief filed late on Wednesday, the U.S. Justice Department called the order an "integral part" of the president's efforts "to address this nation's broken immigration system and the ongoing crisis at the southern border."

The lawsuit filed in Seattle has been progressing more quickly than the four other cases brought over the executive order. It has been assigned to Coughenour, an appointee of Republican former President Ronald Reagan.

The judge potentially could rule from the bench after hearing arguments, or he could wait to write a decision ahead of Trump's order taking effect.

Under the order, any children born after Feb. 19 whose mothers or fathers are not citizens or lawful permanent residents would be subject to deportation and would be prevented from obtaining Social Security numbers, various government benefits and the ability as they get older to work lawfully.

More than 150,000 newborn children would be denied citizenship annually if Trump's order is allowed to stand, according to the Democratic-led states.

Democratic state attorneys general have said that the understanding of the Constitution's citizenship clause was cemented 127 years ago when the U.S. Supreme Court held that children born in the United States to non-citizen parents are entitled to American citizenship.

The 14th Amendment was adopted in 1868 following the Civil War and overturned the Supreme Court's notorious 1857 Dred Scott decision that had declared that the Constitution's protections did not apply to enslaved Black people.

But the Justice Department in its brief argued that the 14th Amendment had never been interpreted to extend citizenship universally to everyone born in the country, and that the Supreme Court's 1898 ruling in United States v. Wong Kim Ark concerned only children of permanent residents.

The Justice Department said the case by the four states also "flunks multiple threshold hurdles." The department said that only individuals, not states, can pursue claims under the citizenship clause, and that the states lack the necessary legal standing to sue over Trump's order.

Thirty-six of Trump's Republican allies in the U.S. House of Representatives on Tuesday separately introduced legislation to restrict automatic citizenship to only children born to citizens or lawful permanent residents.