WASHINGTON (AP) – Guantanamo Bay detainees may not challenge their detention in American courts, a federal appeals court said Tuesday in a ruling upholding a key provision of a law at the center of President Bush’s anti-terrorism plan.

The Court of Appeals for the District of Columbia Circuit ruled 2-1 that civilian courts no longer have the authority to consider whether the military is illegally holding foreigners.

Barring detainees from the American court system was a key provision in the Military Commissions Act, which Mr. Bush pushed through Congress last year to set up a system to prosecute terrorism suspects.

Attorneys for the detainees immediately said they would appeal the ruling to the Supreme Court, which last year struck down the Bush administration’s original plan for trying detainees before military commissions.

“We’re disappointed,” said Shayana Kadidal of the Center for Constitutional Rights. “The bottom line is that according to two of the federal judges, the president can do whatever he wants without any legal limitations as long as he does it offshore.”

A spokesman for the Justice Department, which was expected to seek dismissal of hundreds of detainee cases pending in federal court, praised the decision.

“The decision reaffirms the validity of the framework that Congress established in the MCA permitting Guantanamo detainees to challenge their detention” through military hearings coordinated by the Defense Department,” said spokesman Erik Ablin.

Under the commissions act, the government may indefinitely detain foreigners who have been designed as “enemy combatants” and authorizes the CIA to use aggressive but undefined interrogation tactics.

But most criticized by Democrats and civil libertarians was a provision that stripped American courts of the authority to hear arguments from detainees who said they were being held illegally.

Attorneys argued that the detainees aren’t covered by that provision and that the law is unconstitutional.

“The arguments are creative but not cogent. To accept them would be to defy the will of Congress,” Judge A. Raymond Randolph wrote.

On Tuesday, a spokeswoman for Democratic Senator Leahy, Democrat of Vermont and the chairman of the Judiciary Committee, said he would accelerate efforts to pass a revision to the law that would restore detainees’ legal rights.

Such a provision, introduced by Mr. Leahy and then-Judiciary Chairman Senator Specter, Republican of Pennsylvania, narrowly failed last year on a 48-51 vote.

“The Military Commissions Act is a dangerous and misguided law that undercuts our freedoms and assaults our Constitution by removing vital checks and balances designed to prevent government overreaching and lawlessness,” Mr. Leahy said in a statement.

American citizens and foreigners being held inside the country normally have the right to contest their detention before a judge. The Justice Department said foreign enemy combatants are not protected by the Constitution.

Judge Randolph and Judge David B. Sentelle ordered that the hundreds of cases pending in the lower courts be dismissed.

Judge Judith W. Rogers dissented, saying the cases should proceed.

“District courts are well able to adjust these proceedings in light of the government’s significant interests in guarding national security,” Judge Rogers wrote.

But Jonathan Hafetz, an attorney at the Brennan Center for Justice, said the ruling sends the wrong message about justice to American citizens and the international community.

“It’s a terrible ruling that contradicts centuries of Anglo-American history and allows the indefinite detention of innocent people without charge or judicial review,” he said. “It also allows for detention based on evidence gained by torture.”