A noon-hour bank robbery created a commotion at a busy downtown corner yesterday as the arrival of police and the sudden popping of balloons at an outdoor fashion show sent a swarm of onlookers and passersby diving for cover in the belief that gunshots were being fired.

The episode began about 12:10 p.m. when a man entered the First American Bank branch at Connecticut Avenue and L Street NW, pointed a handgun at a teller and demanded money.

The teller complied, and also set off a silent alarm that summoned police. An outdoor fashion show sponsored by the Connecticut Connection, also located at the busy intersection, was under way as police responded to the robbery call. Persons costumed as clowns distributed balloons.

Police, apparently believing that the robber had fled into a nearby building, cordoned off the area. One officer, according to a witness, told those watching the show to leave or take cover. Spectators heard a helicopter overhead.

Then the balloons began to pop, suggesting the sound of gunfire, and there was a shout that the gunman had been spotted. There were loud voices, pushing and shoving and darting for cover. The excitement, in the words of a witness, for a few moments approached the level of panic.

Police said their search for the bandit was unsuccessful.