John Dickenson saw an incredible sight when he stepped out of his office at the IBM building yesterday morning. Across the lobby, a car had smashed through the glass doors, and near the car stood a masked man, pivoting in a semicircle, coolly blasting away with an automatic weapon.

"He held the gun at his hip," Dickenson recounted hours later, after he had escaped with the help of police. "He started shooting to his right. He was just moving one step at a time." The next thing Dickenson knew, the gunman had swung the gun around in his direction and fired. A bullet struck a colleague standing right behind him.

A gray and quiet Friday morning in Bethesda had been transformed into one of chaos and confusion for 600 employes of IBM. "There's a man with a gun," office workers began screaming, some scrambling for safety outside, others locking themselves in their offices. "It's a man with a gun. Run! Get out!" Their shouts were punctuated by a steady crack of gunfire.

Dickenson ran down the hallway away from the gunman and barricaded himself inside an office with 15 other workers. The companion who had been struck by the bullet struggled to another office. He hid under a desk until police arrived 45 minutes later and sent him by ambulance to Suburban Hospital, where he was rushed into surgery.

Another of Dickenson's coworkers, Chris Shaffer, yelled to secretary Tammy Hill and helped her scamper down a back emergency exit. They ran out of the building and dove right through a hedge. "We didn't know if he was maybe still behind us," said Hill, 35. "Everybody just started running. Nobody looked back." As soon as they were safely outside, Shaffer and Hill stopped a car and hitched a ride to a nearby supermarket, where they phoned police.

James Floyd, a vending machine attendant, was loading a cart of soda into the elevator when the shooting began. He never saw the gunman. "When I got out of the elevator in the main lobby , I saw that a car had crashed through the front doors," recalled Floyd, 25. "Then I heard shots." He dropped his money bag and ran with the cart into a nearby supply room.

"I was hysterical," Floyd said. "I was scared." He placed the cart and a trash can in front of the supply room door and picked up the telephone, frantically trying to reach other offices in the building. Nobody was answering. He tried the police emergency number, 911, but got a busy signal. Finally, he reached his employer. Floyd said he heard "about 20 rounds of ammunition going off about every five minutes or so" for the next 20 minutes.

"I heard shouts, and I thought it was the gunman's voice. I couldn't quite make out what he said. It was 'move' or something like that. Every time I heard him say 'move,' I heard shots fired."

About 45 minutes later, the police appeared outside the supply room door and asked Floyd to open up. He peered out, he said, but slammed the door shut again when he saw guns. Then he heard the crackling of walkie-talkies and was confident that it was the police outside, not the gunman.

"I opened the door again and saw about 20 policemen, and they yelled at me to run out of the building," he said. Floyd sprinted out the south entrance, past the bronze-colored Lincoln Continental that the gunman had crashed through the glass doors.

While Floyd and Dickenson and dozens of other workers hid in offices throughout the square, three-story building, many other employes apparently were unaware of the gruesome scene unfolding in the lobby. Christine Bradley, 21, and her sister Evelyn Carney, 34, who stock the building's bathrooms, were in the basement listening to soul music on the radio. They said they could not hear the fire alarms that had been set off because their office was next to a noisy engineers' station.

"I heard the radio say something about a sniper at IBM," Bradley said. "I turned it up, but then I said to my sister that it must have been a different IBM." She decided to check with her brother, who worked on the loading dock. "When we went up to look," she said, "we saw cops and broken glass, and the cops just said to keep running and don't look back." Two hours later, the sisters found their brother outside, alive and safe.

The hundreds of IBM workers who managed to escape gathered in nearby office buildings and spent the afternoon retelling similar stories of panic: how they first thought the sounds they heard were firecrackers or construction work (which was being done in the building); how they learned of the gunman on the loose, how they scrambled for safety. Corporate officials roamed outside the building trying to determine who was safe and who was still inside with the gunman. They compiled lists of workers who had been located. Sighs of relief greeted each new arrival. One division manager stayed in telephone contact with a group of 12 or more workers who had locked themselves inside the high-security computer area near the basement.

There were acts of bravery throughout the ordeal. John Williamson, the building director, unarmed and uncertain about where the gunman lurked, walked through every part of the building, opening locked doors so that employes could escape. Said David Lawson, a systems programmer: "John Williamson must be the greatest guy in the world."

Donald Miller, who works in the products scheduling division, also served as a brave messenger, running through the hallways to inform coworkers that a man was in the building with a gun. He and his wife, Barbara, who live in Damascus, both escaped. Later, an unidentified woman walked up to Miller, shook his hand, and said: "Oh, thank you, Don. Thanks a lot."

As the afternoon wore on, more of the 600 employes who had begun work that day were rescued. Peggy Finch was one of five workers who locked themselves in an office near the word processing center for three hours before the police rescued them. The four women in the group left their high heels in the office so they could make a swifter escape--and found themselves walking barefooted across a lobby strewn with broken glass. When they reached safety outside the building, they were taken in Montgomery County school buses to a nearby fire station.

The scene outside was as somber as the weather. Dozens of ambulances and police cars lined the streets near the building. Rumors abounded about the name of the gunman, the number of victims. Every so often a police spokesman held forth with the reporters on a sidewalk near the building. Television crews perched their cameras on nearby rooftops. Everyone was waiting for the final scene, hoping that it would be as peaceful as the opening was violent