In an age of terrorism and assassination, a small, private army of civilian security agents is patrolling swamps and jungle-like underbrush around America's $10 billion space shuttle.

The agents, some armed with automatic weapons and dressed in jungle fatigues, were hired by the National Aeronautics and Space Administration to protect the shuttle and were trained by the Wackenhut corp., an international security firm headquartered in Coral Gables, Fla.

A Wackenhut spokesman said, "No comment," when asked if specific threats had been made against the huge and explosive space shuttle, Columbia. Security clearly is tightened far beyond that considered normal for earlier manned flights.

Knowledgeable sources here said the shuttle is unusually vulnerable to attack. "One well-placed shot from a high-powered rifle could bring her down," an industry official said.

The Columbia's most vulnerable part is its huge external fuel tank, which is to be filled with 1 1/2 million gallons of liquid hydrogen and oxygen just before liftoff early Friday. That extremely combustible mixture is in a tank whose walls are almost as thin as an eggshell and easily penetrable by a high-velocity bullet.

Such a shot could disable the craft or even cause it to explode while the two astronauts are aboard. Because the fuel is so volatile, filing the tanks is the last major task before the astronauts board and the final countdown begins.

Nine days after an assassination attempt on President Reagan and with international terrorism increasing, NASA officials are unusually nervous.

"Before, we had campus unrest," said Charles Buckley, chief of security at the Kennedy Space Center. "Now, we have the age of terrorism."

Several vistors here, including some of the 4,000 reporters expected to cover the launch, have reported unnerving confrontations along the edge of the woods inside the space center. Armed, stony-faced men have emerged suddenly from the trees to turn away inadvertent intruders.

Wackenhut, which provides everything from routine security surveillance for businesses to elaborate protective measures for other civilian government agencies, has provided security at the Kennedy Space Center since its construction in 1964. The increased surveillance has added an almost surreal, occasionally frightening, element to this launch.

Richard Wilson, a Wackenhut senior vice president, said the armed guards are "a pretty elite group." He would not say how or where the security forces were trained.

Wilson said Wackenhut also provides fire protection and crash rescue operations for NASA. He said the firm has 20,000 employes worldwide, and protects such installations as nuclear sites and the nuclear test range outside Las Vegas.

Many of the security guards, Wilson said, are former law-enforcement officers or veterans of military operations in Vietnam.

For the first time in the history of America's manned space program, the shuttle has a military as well as civilian role. A Defense Department spokesman said security also has been increased at nearby Cape Canaveral Air Force station, a small installation on the edge of the space center.

The Defense spokesman said civilian guards are being used near the shuttle because the NASA facility is a civilian installation. The space agency never has used military guards.

Buckley said armed guards are patrolling access roads to the launch pad, up to the edge of the "impact limit lines almost three miles from the launch pad" and in unidentified "security buffer zones."

Buckley said the patrols are working 24 hours a day until launch. They have at least two German shepherd attack dogs trained to smell gunpowder and other explosives.

Security guards had patrolled earlier launches, but not to this extent. None had ever been known to carry automatic weapons.