Medical students living in off-campus housing in the hills of Grenada were surrounded by Cuban troops during the height of the fighting on the tiny island nation.

"They could have wiped us out at any time," said Tom Greisamer of San Diego, a retired Air Force captain who flew gunships in Vietnam. "I don't know why they chose not to. I don't know. Our guys were just incredible in getting us out."

Greisamer, now a medical student at St. George's University School of Medicine, was one of 383 students and faculty members evacuated at that time from the island by U.S. Air Force transport planes.

Greisamer and a fellow student, Phil Gianelli of New York, recounted hours of waiting for rescuers from the U.S. invasion force to reach them as full-scale hostilities roared around them Tuesday and Wednesday.

Gianelli said, "It became increasingly tense for us. We didn't know how we would be contacted . . . . There were 50 students and faculty in the area I lived in and we were stranded in those hills. We were picked up last night by a school bus and taken to the campus. We were told to lie face down until summoned by U.S. Rangers. We ran to copters on the beach, under fire . . . . I'm a wreck."

With sniper fire ringing above them, he and other Americans were herded onto the helicopters, ferried to the Grenada airport and flown to the Charleston Air Force Base, shaken but more than glad to be here.

Greisamer said that shells fired toward Cuban or Grenadan positions by U.S. vessels offshore were landing "less than 50 meters from our building on the campus."

Gianelli told of seeing a U.S. helicopter go down, apparently hit, as it was firing on the well-fortified Richmond Hills political prison. He also reported that fire from a U.S. missile cruiser obliterated Fort Rupert, a military installation in St. George's harbor.

"If I can use the term, the Grenadan people are very much in favor of the U.S. emancipating them . . . . They can't wait to parade down the streets with Americans. They hate the Cubans and Soviets . . . . My neighbors couldn't wait to get their country back," Gianelli said.

Five other evacuees, unidentified U.S. soldiers, arrived here Wednesday night and were listed in good condition at a Navy hospital. A spokesman said the GIs were rejecting all interview requests. Two of the five suffered leg fractures; three had gunshot wounds.

After the arrival of six flights from Grenada by 6 a.m., the flow of evacuees stopped abruptly today without explanation. Military Airlift Command officials said they had no details but expected at least one more flight tonight.

Gianelli and Greisamer agreed with other arriving students' versions that although they experienced few or no direct threats before the invasion Tuesday, political tension, a 24-hour-curfew and their isolation on the two medical school campuses created intense anxiety.

"There was an air of repression," Greisamer said. "In effect, it was martial law. They were taking Grenadan citizens out of their homes . . . . Wives were crying for fear they would not see husbands again. Fear of physical threats? Yes, there was an awful lot of intimidation: you go out, you get shot."

Gianelli said that on Monday, hours before the invasion, three armed Grenadan soldiers arrived in his neighborhood, parked their truck behind a bush and watched his house through the night.

He said that his mother, Frances Gianelli of New York, visiting him since early October, sat on the balcony and stared back at the soldiers while he slept. Early Tuesday, he said, his mother called, "I hear planes."

The invasion had begun, as U.S. aircraft flew overhead and drew intense antiaircraft fire, he said. "I went out on the balcony and I could see tracers. I figued this was the real thing. But I worried because one of the larger Grenadan guns was on a hill behind where I lived."

Like other students arriving here, Gianelli and Greisamer gave U.S. troops high marks for their rescue work and agreed that President Reagan's decision to invade most likely had saved them from future harm.

Although they would provide few details about the evacuation, Air Force officials here, apparently unaccustomed to praise, handed out copies of a student-faculty statement lauding the U.S. defense forces.

For her part, Frances Gianelli was saying little about her vacation in the Caribbean sun. She huddled under a blanket at the Air Force terminal and sipped coffee this morning. "I just went to see my son . . . and then this," she said.