WASHINGTON (Reuters) - Supreme Court justices on Wednesday questioned whether terrorism suspects at Guantanamo Bay have constitutional grounds for challenging their years-long detention, in a major case spotlighting the tarnished U.S. human-rights record.

The lawyer representing 37 Guantanamo inmates argued before the highest U.S. court that Congress wrongly stripped the prisoners in 2006 of their habeas corpus rights to seek a judicial review of their detention.

Justices asked whether Congress had created an adequate alternative for prisoners to contest their detention, and whether the Constitution protects the rights of foreigners held by the United States outside the country.

“If our law doesn’t apply, this is a law-free zone,” detainee attorney Seth Waxman told the court, referring to the prison at the U.S. Naval base on Cuba.

Centuries of U.S. and English legal tradition were examined in the court’s oral arguments. The case is being watched by governments and activists around the world, who say President George W. Bush has overreached his powers and trampled on human rights in the war on terrorism he launched after the September 11 attacks.

A handful of protesters outside the court wore orange prison-style jumpsuits with black hoods and chanted “equal justice under law, defend the Constitution.”

Many prisoners have been kept at Guantanamo for nearly six years while courts and the military wrangled over their cases, Waxman told the court. “The time for experimentation is over.”

The Bush administration contends the detentions are lawful, humane and necessary in a new-style war on terrorism.

Justice Department Solicitor General Paul Clement, arguing the administration’s case, said the prisoners have more rights now than under original U.S. habeas corpus law in 1789. “This is a remarkable liberalization,” he said.

ZEROING IN

The court’s arguments ran about 22 minutes over the scheduled one hour. A decision is expected by summer.

Chief Justice John Roberts zeroed in on an issue some have said would be the crux of the case, whether the 2006 law which allowed for a limited review by a federal appeals court in Washington offered the detainees an adequate alternative to make their case in court.

“So our judgment in this case depends on whether we agree with you, or the government, that the (alternative) procedures ... are meaningful?” Roberts asked Waxman, who said yes.

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Justice Anthony Kennedy, a swing vote, asked whether the appeals court in Washington should not first rule on whether the alternative process was adequate. The appeals court did not rule on the issue when it decided against the detainees in the case the court heard on Wednesday.

The high court has ruled against the administration in two previous Guantanamo cases and one other terrorism case. But Congress adopted new measures including the 2006 law aimed at keeping such cases out of court by removing the prisoners’ habeas corpus rights under federal law.

This is the first time the court is considering whether the Guantanamo prisoners have those rights under the Constitution itself. But some justices suggested the court’s earlier rulings meant they do. “We passed that point, haven’t we?” Justice David Souter asked.

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The Guantanamo prison opened in January 2002 after the U.S.-led invasion of Afghanistan. The administration contends the naval base, on land leased from Cuba, is outside U.S. territory so constitutional protections do not apply.

Justice Antonin Scalia pressed Waxman for historical examples of habeas corpus rights extending to foreign prisoners held outside the United States or England, the source of U.S. habeas corpus law. He disputed examples cited by Waxman.

Most of the 305 prisoners at Guantanamo have been confined for years without charges and many have complained of abuse. Bush has acknowledged the prison’s damage to the U.S. image and said he would like to see it closed eventually.

About 470 prisoners have been released, and the United States said it intends to try 60 to 80 of those still in detention.

Also on Wednesday, a Guantanamo tribunal began a hearing in the case of Salim Ahmed Hamdan, an accused guard for Al Qaeda leader Osama bin Laden.