By Jonathan Stempel and Terry Baynes

WASHINGTON, Oct 29 (Reuters) - The U.S. Supreme Court on Monday considered who has a right to challenge government eavesdropping on conversations between people in the United States and outside the country in a case touching on federal efforts to fight terrorism.

Since the Sept. 11, 2001, attacks, the Supreme Court has been reluctant to intervene in national security and intelligence-gathering procedures of the country's executive branch, and the government has said it needs flexible surveillance power to help prevent strikes by foreign militants.

Lawyers, journalists and human rights groups including Amnesty International and Human Rights Watch said the Foreign Intelligence Surveillance Act (FISA) - the law allowing the surveillance - is too broad and could subject millions of people to monitoring without a warrant.

They said this violated the protection against illegal searches and seizures under the Fourth Amendment of the U.S. Constitution.

Arguments heard by the court on Monday addressed not the monitoring itself, but whether the objectors had the right to sue at all.

"Is there anybody who has standing?" Justice Sonia Sotomayor asked Solicitor General Donald Verrilli, who argued on behalf of the government to uphold the law. "If there was a constitutional violation in the interception ... no one could ever stop it until they were charged with a crime, essentially."

Last year, the 2nd U.S. Circuit Court of Appeals in New York let the case go forward, based on the plaintiffs' fear of surveillance and the cost of trying to avoid it.

But in its appeal, the government said that because the surveillance is secret, the challengers could not show current harm or that future harm was "certainly impending."

'CASCADE OF SPECULATION'

"They are asking you to invalidate a vitally important national security statute" over claims that "depend on a cascade of speculation," Verrilli said.

Adopted in 1978, FISA required the government to submit a surveillance application to a special court for each person outside the country who it was targeting.

After the Sept. 11 attacks, President George W. Bush authorized the National Security Agency's use of warrantless wiretaps to find people with ties to the al Qaeda network and other groups.

He ended that program in 2007, but Congress the next year reinstated parts of it. The government may now seek court permission to conduct mass surveillance, without identifying specific targets, merely by saying it plans to monitor foreigners outside the country to gather foreign intelligence.

Jameel Jaffer, arguing on behalf of the statute's challengers, said the law effectively allows "dragnet surveillance" that forces people who are in touch with potential government surveillance targets to avoid using phones and email to communicate, and resorting to more expensive means.

Some justices suggested that the harm to the challengers might not be so great. "The question is whether or not your clients' conversations can be picked up in an incidental way," Chief Justice John Roberts told Jaffer.

Justice Samuel Alito expressed skepticism over Jaffer's position, saying it could lead to a "bizarre" outcome in which a suspected top terrorist could escape surveillance by hiring American lawyers who can use the statute's protections.

Jaffer said he did not mean to suggest that scenario, but that the government's position required special steps.

"If all the government were to do, at this point, is to say secretly to a judge, 'We're not actually going to use this against plaintiffs,' plaintiffs would have to take the same measures they're taking right now," Jaffer said. "And they would be injured in exactly the same way."

Six former U.S. attorneys general submitted a brief supporting the government, fearing a floodgate of litigation that would risk exposing state secrets.

A decision is expected by the end of June.

The case is Clapper et al v. Amnesty International et al, U.S. Supreme Court, No. 11-1025.

For Clapper, Director of National Intelligence, et al: Solicitor General Donald Verrilli.

For Amnesty International et al: Jameel Jaffer of the American Civil Liberties Union.

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