By Basil Katz

NEW YORK (Reuters) - The U.S. government on Monday asked for an emergency stay of a federal judge's ruling against a section of a law that activists and journalists had said could permit their indefinite military detention.

The U.S. Justice Department said the ruling against a portion of the National Defense Authorization Act's "Homeland Battlefield" provisions would hurt America's ability to fight wars overseas.

The ruling was issued last week by Judge Katherine Forrest in favor of a group of activists and reporters who said they feared being detained under the law signed by President Barack Obama in December.

The provision authorizes indefinite military detention for people deemed to have "substantially supported" al Qaeda, the Taliban or "associated forces."

The Justice Department on Monday asked the 2nd U.S. Circuit Court of Appeals in New York for an "immediate administrative stay" of Forrest's ruling.

"The district court's overbroad, worldwide injunction is erroneous as a matter of law and threatens tangible and dangerous consequences in the conduct of an active military conflict," the government said.

Prior to Monday's emergency filing, the government had lodged a non-urgent appeal last Thursday and asked Forrest for an immediate stay of her injunction pending the higher court decision on the case.

Forrest, however, denied that request and said she would rule on Wednesday whether to suspend her injunction from then on.

The judge's 112-page opinion on September 12 extended a temporary injunction issued in May. The case stems from a January lawsuit filed by former New York Times war correspondent and Pulitzer Prize-winner Chris Hedges and others.

The plaintiffs said they had no assurance that their writing and advocacy activities would not fall under the scope of the provisions.

Government lawyers on Monday repeated their earlier defenses of the law but addressed the appeals court with renewed urgency.

The United States argues that the plaintiffs had no basis to fear being locked up for their activities, and that the judge's order interfered with the president's powers at a time of war.

"The court has placed the military in a difficult and burdensome position rife with confusion," the government said.

Lawyers for the plaintiffs responded Monday that the United States was not permitted to ask the appeals court to intervene until Forrest had issued a decision on the government's motion to stay the injunction.

The cases are Hedges et v. Obama, U.S. District Court for the Southern District of New York, No. 12-cv-331 and Hedges et v. Obama, 2nd U.S. Circuit Court of Appeals, No. 12-3176.

(Editing by Eric Beech)