Former National Security Council aide Oliver L. North yesterday gave the Iran-contra committees a special presentation of the slide show he had used when he sought to convince potential supporters of the Nicaraguan rebels that the contras are a bulwark against "the Soviet threat in this hemisphere."

But because security considerations prevented the lights from being turned off in the Senate hearing room, the panel members -- and millions of viewers watching on national television -- had to settle for the sound without the sight.

Instead of projecting the slides onto a screen, the Marine lieutenant colonel held them up one by one and recited his well-practiced pitch for support of President Reagan's Nicaragua policy.

The hearings had previously heard suggestions that North might have used the briefing as part of improper efforts to entice wealthy conservatives to contribute money to the contras.

North denied ever engaging in contra fund-raising, and administration backers wanted the briefing performed before TV cameras in the belief it would show the contras favorably and rally public support for them.

North's presentation yesterday was an expanded and updated variation on material -- mostly aerial photographs of alleged military installations and weaponry strung together by anticommunist rhetoric -- that the administration has been showing to audiences around the country for at least five years.

The briefing seeks to establish that Nicaragua's Sandinista government is dominated by communists who want to turn the country into a totalitarian state on the model of Cuba and the Soviet Union. Its other main assertion is that the Sandinistas are engaged in a massive military buildup threatening other countries of Central America.

North described Nicaraguan Defense Minister Humberto Ortega as "a mirror image of what is going on in Cuba."

"The minister of defense in Cuba is the brother of Fidel Castro. The minister of defense in Nicaragua is the brother of el Presidente Daniel Ortega," he said, pausing to let his listeners ponder that point.

Some of the aerial photos in North's presentation show what he charged were Nicaraguan military installations built according to an alleged Cuban model, airfields with runways lengthened to handle Soviet jets, and buildups of Soviet equipment inside Nicaragua.

The administration has been using versions of these photos to press its case since March 1982, when it invited the press to a lengthy State Department briefing by Adm. Bobby R. Inman, then deputy director of the Central Intelligence Agency.

In the years since, the administration has developed a far-reaching public diplomacy program in which North and other senior officials involved with Central America have given similar briefings to audiences of reporters, business and civic leaders and others that the administration wanted to impress.

North was especially active in making his presentation to groups that were invited to the White House under the Central American Outreach program conceived by former presidential aide Faith Ryan Whittlesey.

North, in an apparent attempt to counter charges that the contras commit human rights violations, said the blame for rights violations rests with the Sandinistas.

He showed pictures of a Pentacostal minister who he said was badly injured when the Sandinistas burned his church, and of a contra fighter, described as a 57-year-old coffee farmer, whose "entire family was murdered by the Sandinistas because they gave water to a passing contra patrol."

Aryeh Neier, head of Americas Watch, which monitors human rights violations in Latin America, said yesterday he knew of no reports or documentation of the second incident.

He said the first apparently referred to Prudencio Baltodano, a Pentacostal minister who, in the opinion of Americas Watch, was tortured and mutilated by Sandinista troops because they suspected he was helping the contras.

While acknowledging that the Sandinistas have been repressive toward domestic dissent, Neier said, "Our view is that there has been no systematic pattern of atrocities by the Nicaraguan government. But it can be documented that both sides -- the Sandinistas and the contras -- have at times engaged in incidents that are fairly described as atrocities."