JERUSALEM, AUG. 30 -- The treason and espionage trial of Mordechai Vanunu, the former nuclear technician who sold Israeli atomic weapons secrets to a London newspaper last fall, opened today behind closed doors and amid tight security.

The authorities barred journalists from the district court building here, boarded up the windows of the room where the trial is being held and hammered canvas over the back entrance of the room in an attempt to fully seal off the proceedings from public view. Journalists were warned that their reports on the trial could be subject to military censorship.

Vanunu was kept from public view and brought here in a blue police van that entered the compound nearly two hours before the trial started. The van's windows had been whitewashed, apparently in an effort to avoid repetition of an incident last December when Vanunu embarrassed the authorities by alleging -- in a message scrawled on his palm pressed against the window of a van that transported him -- that he had been kidnaped by Israeli agents in Rome and brought here against his will.

Israeli officials have denied that Vanunu was abducted but have offered no explanation of how he was returned to this country.

Officials have said the extraordinary security measures surrounding the trial are necessary to protect state secrets, but Vanunu's lawyer, Avigdor Feldman, disputed that claim.

"The state has a history of paranoia on the {nuclear} issue," Feldman told reporters outside the courthouse this afternoon. "The security is exaggerated. There's no reason to hide him and not let the public see his face. I think that all these security regulations put a stigma on my client that he is a dangerous person, someone the public should not see."

Feldman said he made two motions to the three-judge panel at the session, which lasted five hours today. The defense moved, first. that the charges be dismissed because Vanunu was brought to Israel illegally, second, that the defendant's confession be thrown out because it was made under duress. Family members have contended Vanunu has been held in solitary confinement and denied regular prison privileges for long periods.

Four prosecution witnesses appeared today, according to a statement released by the Justice Ministry. They were Deputy Police Commander Shimon Savir, head of the unit that investigates serious crimes, and three members of the Shin Bet, Israel's internal security service. The judges have indicated they will rule on the defense motions after hearing the prosecution's case, Feldman said. The trial is expected to last two weeks.

Feldman said he had asked the judges to open at least parts of the trial and said he expected a ruling from them later this week.

Vanunu faces two counts of treason and espionage. He is accused of conveying secret information that could assist enemies of the state of Israel and of doing so with the intent of harming state security. If convicted, the 33-year-old Moroccan-born Israeli could face the death penalty, although his prosecutors have indicated they will seek a life sentence instead.

Relying on information smuggled out of the country by Vanunu, the Sunday Times of London reported last fall that Israel has been building nuclear weapons for two decades and has a stockpile of up to 200 warheads, making it the world's sixth largest nuclear power.

Israel's nuclear capability has long been an open secret here, but the government's policy is to maintain deliberate ambiguity. Officials say only that Israel will not be the first to introduce atomic weapons into the region.

The Sunday Times disclosures embarrassed Israel's security agencies, which were faulted for allowing Vanunu to continue working in sensitive facilities at a Dimona nuclear power plant for several years despite Vanunu's outspokenly left-wing views and then allowing him to leave the country with a suitcase full of photographs and data.

Vanunu disappeared from London shortly before the Sunday Times article appeared. Relatives later claimed he had been lured to Rome by an Israeli Mossad agent calling herself "Cindy" who promised him sex. Once in Rome, the family said, he was abducted, drugged and brought back to Israel aboard a ship.

At first the Israeli government denied knowledge of Vanunu's whereabouts, but officials admitted several weeks later that he was in custody. They have refused to discuss how he was returned to Israel except to say that no British laws were broken. They did not mention Italian law.

Among those who were denied entry to the trial today were Vanunu's younger brother Asher and Menno Kamminga, a representative of Amnesty International, the London-based human rights organization, who in a printed statement expressed concern over the conditions of the trial and imprisonment.