There were no reports of significant damage during the quake, which struck at 12:15 a.m. local time
But the independent B-92 Radio said one man died of a heart attack during the quake near the town of Valjevo. At least 17 people were injured, many of them tumbling down darkened staircases as they fled into the streets.
Officials said the quake was centered near Cacak, 55 miles south of Belgrade.
In central Serbia, authorities warned people to avoid drinking tap water because sewer systems may have been damaged. Schools were closed.
Some who called into Belgrade radio stations Wednesday asked whether NATO had attacked. NATO has threatened possible military action unless President Slobodan Milosevic halts his crackdown on ethnic Albanian separatists in the southern province of Kosovo.
In neighboring Bosnia-Herzegovina, people fled into the streets of the capital, Sarajevo, despite a heavy downpour. The quake was also felt in parts of Croatia.