A man waving a 12-inch kitchen knife and shouting demands for his birth certificate and other documents terrorized clerks and citizens in Alexandria City Hall briefly yesterday afternoon before a city employe felled him with a body block and wrested the knife away.

The employe, Robert Myers, a heavyset man of 40 who counsels ex-convicts as part of a city program, suffered a cut on his left palm and a bite on his arm, and was sent to a hospital for a tetanus shot.

Police said they arrested Larry Purcell Garnett, 23, of 110 Baggett Place, Alexandria, and charged him with felonious assault.

The incident began at 4:15 p.m. when a man approached telephone operator Elizabeth Heizer in City Hall lobby, leaned over the counter behind which she works and struck her desk top with the knife.

"He came around to the gate here and started shouting, 'Where's my birth certifcate and my high school diploma?' " she said. Heizer retreated to the rear wall of her booth and told him the documents were available at another building.

He moved to come through the gate into her booth but fumbled with a latch, Heizer said.

At this point Myers, who was in City Hall to pick up an expense advance for a business trip, approached to see what the noise was about. Seeing the man, he picked up a metal post used to support a felt rope and waved it at the man to try to distract him from Heizer.

The man advanced at him. Myers retreated, he said, and the man shouted he was tired of bureaucracy and was being thrown out of his home and denied services he deserved.

He then turned his attention to a chest-high counter where citzens line up to pay taxes, license costs and and other fees to city Treasury Division clerks on the other side. He went over the counter "with one huge leap and with a knife coming outward," said account clerk Jean Browne.

The dozen clerks who work in the green room behind the counter scattered in panic through two doors as he continued to wave his knife, witnesses said.

Myers leaped over the counter too and rushed the man, now at a door that a woman had locked from the other side. "I just put a linebacker move on him," Myers said. The man collapsed and Myers wrested the knife away. Police arrived shortly afterward.

Myers, who managed a cocktail club before becoming a counselor last year with the Virginia Cares program, which seeks jobs for ex-offenders, finally was handed his check after the incident. "This is what I was waiting for," he joked.

Heizer, filling out a police report, stopped to thank him for his help.