The Reagan administration, seeking to swing public opinion behind its Central America policies, yesterday staged a display of blowups of aerial photographs to bolster its charges that Nicaragua is engaged in a massive military buildup that poses a threat to its neighbors.

The photographs, projected onto a giant screen in the darkened large auditorium of the State Department, were the highlights of a lengthy press briefing given by Adm. Bobby R. Inman, deputy director of the Central Intelligence Agency, and John Hughes, deputy director of the Defense Intelligence Agency.

As described by Hughes, the photographs purported to show Nicaraguan military installations built according to an alleged Cuban model, airfields with runways lengthened to handle Soviet Mig jets that the United States claims are earmarked for Nicaragua, and Soviet tanks and artillery in place at some of these installations.

The two officials also showed a series of before-and-after photographs showing Indian villages near Nicaragua's border with Honduras that allegedly were burned by Nicaraguan authorities. The administration has charged Nicaragua with repression and forced relocation of the Miskito Indians and other tribes that lived in the area.

Except for the photographs, collected by unspecified aerial reconnaissance methods, the briefing was largely a reiteration of information that the administration previously made public.

Its main point, which Inman and Hughes tried to drive home with a drumbeat of tough rhetoric, was the allegation that Nicaragua's revolutionary, Sandinista-controlled government, with Cuban and Soviet help, is building an army whose size is far larger than what the country needs for legitimate self-defense. Inman said the intelligence community believes Nicaragua's goal is to create a standing army of 25,000 to 30,000 and a ready-reserve militia of between 100,000 and 150,000.

He added that while the purpose wasn't clear, he believes the "pattern of Cuba" is being repeated in Nicaragua and that "the military infrastructure is there to turn the country into a Soviet bastion" from which campaigns of political intimidation or outright warfare can be waged against the rest of the Central American isthmus.

"This time, the ocean barriers aren't there," Inman said. "They can move much more easily into Central American countries."

What set yesterday's briefing apart from previous administration attempts to make its case about communist penetration of Central America was the almost theatrical flair with which it was staged. Administration officials also made clear it was only the opening salvo in a major effort to influence public opinion and counter the mounting opposition in Congress and elsewhere to President Reagan's backing of the military-civilian goverment in El Salvador and his hostility to Nicaragua.

The session yesterday was limited to presenting evidence about the Nicaraguan arms buildup and did not deal with the charge made by Secretary of State Alexander M. Haig Jr. that the leftist guerrillas in the Salvadoran civil war are controlled and supplied by such outside forces as Cuba and Nicaragua. Haig's accusation caused several members of Congress who advocate negotiating with the Salvadoran guerrillas and Nicaragua to ask him for proof.

Inman said evidence about the so-called "Salvador-Nicaragua connection" is being presented to Congress in closed briefings that will continue through Thursday. He also said a similar briefing will be given today at the State Department for a bipartisan group of distinguished former government officials, and he added that another press briefing on the subject is likely Friday.

The tone of the administration's approach was set by Inman, who opened the briefing by saying he was "concerned" and "angry" because the public has not been getting "a clear idea of what is concerning us in the intelligence community" and because government officials have been inhibited in responding to congressional interrogation by the need to protect intelligence sources.

For that reason, he continued, CIA Director William J. Casey had declassified the aerial photographs made public yesterday. To explain them, Inman then called on Hughes, whom he described as "the premier photo interpreter in the intelligence community." He also noted pointedly that Hughes had conducted the briefings during the 1962 Cuban missile crisis dealing with aerial reconnaissance of Soviet missile sites there.

Hughes said there are 49 active military garrisons in Nicaragua, 36 of them built since the Sandinistas won power in 1979. He then showed aerial photos of several that he said were built on the Cuban pattern of a rectangle divided into three parts: a motor pool, a barracks area and a training area containing what he contended were "Soviet-model" obstacle and physical training courses.

To emphasize that point, Hughes also displayed an aerial view of a military garrison in Cuba constructed in the same way. As the photos of the different Nicaraguan installations were flashed on the screen, Hughes pointed out what he said were Soviet T55 tanks and artillery pieces, East German trucks and hardened shelters for jet fighters.

He said the Nicaraguans have an estimated 25 T55 tanks equipped with 100-mm. guns, 12 155-mm. howitzers, 12 armored personnel carriers, two heavy-duty Soviet helicopters and amphibious ferry equipment. He added that four airfields are being prepared for Migs that the United States expects to arrive later this year, and he contended that this equipment gives Nicaragua an arsenal more formidable than that of any other Central American country.

In other actions relating to Central America yesterday, the State Department dismissed as one-sided an Amnesty International report accusing government forces of murdering civilians in El Salvador and urged authorities in Guatemala to respond quickly to charges of irregularities in its presidential election last Sunday.

On Capitol Hill, Sen. Mark O. Hatfield (R-Ore.) joined the ranks of Reagan's critics by announcing that he will ask Congress to halt the "mad course" toward a Vietnam-type involvement in El Salvador. But Sen. Steve Symms of Idaho and five other Republican conservatives called on the Senate to reaffirm a 1962 resolution authorizing the use of any means, including military force, to halt Cuban intervention in the hemisphere.