Only the accused, their official counsels and those who have "legal involvement" in the case will be allowed to attend the trial, which opens Thursday in Shiraz, capital of Fars province, the radio said, quoting a statement by the province's justice department.
"With regard to the special circumstances of the case and the possibility of interference in national security and the repercussions of that...the Islamic Revolutionary Court has issued orders that the proceedings be closed," the radio quoted the department as saying.
The suspects were arrested more than a year ago and accused of spying for the United States and Israel. Both countries say the charges are baseless.
The Revolutionary Court, where the suspects will be tried, has no jury. The judge leads the investigation, prosecutes and hands down the sentence and verdict.
If convicted, the suspects could get long prison terms or death sentences. In a similar case three years ago, two Jews were executed at Tehran's Evin prison.
Dozens of Jews including family members of the 13 accused, gathered Wednesday at the Rabeezadeh Synagogue in Shiraz, offering prayers for the defendants.
"I'm sure they are innocent. Even if they are found at fault of a minor incident, we request and hope that the Islamic Republic, with its generosity, will acquit them," said Rahmat Farzam, whose son Ramin, 27, is among the 13 defendants.
"Spies are usually well-off. My son has no money at all, so how can he be a spy?" asked Farzam. He said he was allowed to meet with his son Tuesday for three minutes and found him in good health but dispirited.
Iran, which has been under international pressure to free the Jews or ensure they are tried fairly, maintains religion has no bearing in the proceeding and notes that eight Muslims also have been arrested in the case.
The trial opens amid a serious power struggle between anti-American Islamic hard-liners and President Mohammad Khatami's reformist wing, which favors better ties with the United States. The judiciary is controlled by the hard-liners.
The United States has warned the trial could affect a growing Iran-U.S. rapprochement under way since Khatami's May 1997 election.
Israel, France, Germany, the United Nations and Amnesty International have condemned the arrests or called for a free and fair trial.
The trial coincides with the start of Ashoura, a period when Iran's predominantly Shiite Muslim population mourns the death of its most revered saint.
Jewish leaders had asked that the trial be postponed until after Ashoura, but their request was turned down.
At its height, Iran's Jewish community numbered about 100,000 and was still around 80,000 before Iran's 1979 Islamic revolution. Most left after the revolution, and the population has dwindled to about 25,000.
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