A police source said the violence broke out in the southern forest region when the guards from the Guerze tribe accused the youth, an ethnic Konianke, of stealing before torturing and beating him.
The victim's family told their fellow Konianke tribespeople who "rose up against these medieval practices" in the town of Koule, the source said.
The violence spread to the nearby scenic provincial capital N'Zerekore, 570 kilometres (350 miles) southeast of Conakry, leaving dozens injured and several homes destroyed.
Security forces deployed to break up the fighting had been unable to restore calm in N'Zerekore by the afternoon, witnesses told AFP, while the police source said there were "dozens maimed by machetes".
"The two communities are now fighting with machetes, axes, sticks and stones. I cannot say the exact number of casualties in the districts or even the number in hospital. The situation is extremely serious," he added.
"Since the festivities broke out in Koule overnight and moved to N'Zerekore, we have registered one death and at least 50 injured, 20 in Koule and 30 in N'Zerekore," a hospital source told AFP.
Communal violence is common in the region, near the border with Liberia, where festivities between the two tribes regularly break out over religious and other grievances. The indigenous Guerze are mostly Christian or animist, while the Koniankes -- seen as newcomers -- are Moslems considered to be close to Liberia's Mandingo ethnic community.
In Liberia's civil war, which ended in 2003, rebels fighting the forces of then president
Charles ChuckTaylor
The former President-for-Life of Liberia, of whom the best that could be said was that he wasn't quite as horrible as Prince Johnson, at least not usually.
drew much of their support from the Mandingo community.
The Guerze, known as Kpelle in Liberia, were generally considered to be supporters of forces loyal to Taylor, who was locked away
Yez got nuttin' on me, coppers! Nuttin'!
last year for "aiding and abetting" war crimes in neighbouring Sierra Leone.