Looting has plagued Monrovia since April when fighting between rival factions led the city into chaos. Foreigners and aid agencies evacuated, and militiamen stripped their homes and offices.
A May 27 truce calmed the city, but Saturday night's incident showed few Liberians are confident of lasting peace.
When the shooting began, tens of thousands of people from the Barclay Training Center scrambled across the two bridges leading out of downtown Monrovia.
A soccer game near the training center was canceled as spectators and players ran for cover. Traders in a nearby market grabbed their goods and fled. No injuries were reported.
West African peacekeeping soldiers moved in and fired warning shots. Calm was restored within 15 minutes.
Several factions have struggled for power since Liberia's civil war began in 1989. More than 150,000 people have died in the war, and half the population of 2.3 million has been left homeless.
=A9 Copyright 1996 The Associated Press