By Nate Raymond

Feb 18 (Reuters) - A federal judge threw out on Wednesday an administrative board's decision endorsing the Trump administration's policy of subjecting thousands of people arrested during its immigration crackdown to mandatory detention.

U.S. District Judge Sunshine Sykes in Riverside, California, vacated the decision by the Board of Immigration Appeals after finding the administration had failed to comply with her earlier order declaring unlawful the underlying policy of denying detainees the chance to seek release on bond.

Sykes' Wednesday ruling, in a class action lawsuit that covers migrants nationwide, is more sweeping than decisions by hundreds of other U.S. judges holding the policy is unlawful and ordering detainees to be freed or given bond hearings.

Sykes, appointed by former Democratic President Joe Biden, called the administration's actions "shameless" and accused it of trying to continue its "campaign of illegal action" by still refusing bond hearings despite her prior ruling.

"Respondents have far crossed the boundaries of constitutional conduct," Sykes wrote.

The U.S. Department of Homeland Security and the U.S. Department of Justice, which oversees the board, did not immediately respond to a request for comment.    

Sykes' ruling means the board's decision can no longer be used by immigration judges to deny bond hearings, said Niels Frenzen, a professor at the University of Southern California's Gould School of Law who represented the plaintiffs.

"We hope that DHS and the immigration courts will now comply with the court's orders to provide bond hearings to the thousands of noncitizens who have been arrested," he said in a statement.

Federal immigration law prescribes mandatory detention for "applicants for admission" to the United States, while their cases proceed in immigration courts and they are ineligible for bond hearings.

Bucking a long-standing interpretation of the law, the DHS last year - as part of President Donald Trump's immigration crackdown - took the position that non-citizens already residing in the United States, and not only those who arrive at a port of entry at the border, qualify as applicants for admission.

The Board of Immigration Appeals, which is part of the Justice Department, issued a decision in September that adopted that interpretation, leading to immigration judges nationally employed by the department to mandate detention.

Sykes in a ruling in December declared the DHS policy unlawful but stopped short of vacating the board's decision.

But she said it was clear further relief was needed after Chief Immigration Judge Teresa Riley issued guidance instructing her colleagues that they are not bound by Sykes' ruling and that they should continue following the board's decision.

Those immigration judges are employed by the Justice Department.

Sykes, in Wednesday's decision, criticized DHS for repeatedly and inaccurately suggesting that operations by U.S. Immigration and Customs Enforcement were limited to targeting criminal non-citizens who were the "worst of the worst."

"Maybe that phrase merely mirrors the severity and ill-natured conduct by the government," Sykes wrote. "Even though these press releases might contain an inkling of truth, they ignore a greater, more dire reality."

(Reporting by Nate Raymond in Boston; Editing by Muralikumar Anantharaman, Stephen Coates and Clarence Fernandez)