A two-block-long stretch of grass in front of the D.C. Armory became an Army field hospital yesterday, complete with 17 doctors and helicopters and ambulances ferrying in 40 "victims" who had been burned, some severely, when a tanker truck filled with a flammable liquid overturned in downtown Washington.

But this was not a real disaster. Instead, it was part of an elaborate mock disaster staged by 139 members of the D.C. National Guard to show off their medical emergency capabilities to observers from around the area.

"We're showing how we can play a vital role in the event that a major disaster were to strike our community," said Maj. Gen. Calvin G. Franklin, commanding general of the D.C. National Guard.

The two-hour demonstration had all the drama of a real emergency.

In the imaginary disaster, fumes from the 5,000 gallons of spilled liquid -- an unidentified, but poisonous substance -- seeped into the air shafts of the area's subway system. Several violent explosions sent glass and other debris flying from nearby buildings, hitting passers-by. Flames leaped from building to building, causing hundreds of injuries.

Under these circumstances, according to the guard, the casualties would overload local hospitals.

The guard's answer to such an emergency would be to assist local officials by quickly contructing field hospitals known as MASH units (mobile Army surgical hospitals) that are used in war zones. These medical units, staffed by doctors, nurses and orderlies, would be built near the disaster site and would be accessible to ground transportation and helicopters.

Yesterday's demonstration was designed to show the effectiveness of the plan, guard officials said.

The buzzing of a helicopter, one of four from the 400th Medical Detachment of the guard, signaled the beginning of the demonstration at about 1 p.m. Once the helicopter was on the ground, three-man rescue crews wearing camouflage fatigues rushed forward with stretchers to gather the four "patients" from the chopper's belly.

Other patients began arriving by ambulance, and soon the ground was covered with "patients" writhing in "pain."

Makeup was used to create genuine-looking cuts and burns. Clothing was stained with red dye to resemble blood. Broken legs, arms and backs were set, plastered and bandaged by the dozens of doctors, some of whom were dressed in hospital masks and gowns that covered their Army garb.

The doctors bent over patients, shouting to nurses for needed instruments, equipment and more casualties for treatment. Even a chaplain was on hand to "pray" for the victims.

Eileen Foster, an emergency medical instructor for the state of Maryland, who was an observer, said she was impressed by the demonstration. "It's much different under real circumstances, but I think they did a fine job."