At least 16 people were admitted to hospital yesterday with bullet wounds following the unrest in the city of N'Zerekore, a hospital source told AFP, adding that six others came in with injuries caused by sticks and stones.
In another hospital in Guinea's second-largest city, a source spoke of "around a dozen young people" injured.
It marks the second major outbreak of violence in the run-up to the October 11 polls, after rival factions clashed in the northern town of Koundara in late September. At least 17 people were injured then, according to witnesses and security sources.
Many of the area's shopkeepers belong to the Fulani ethnic group, traditionally loyal to opposition challenger Cellou Dalein Diallo, a local police official told AFP on condition of anonymity.
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"That's what lit the fire," he said, adding that the clashes continued into yesterday.
Local residents confirmed his version of events. "Stones were thrown and there are numerous injuries on both sides," one witness said.
Eight contenders -- including Conde and Diallo -- have been approved as candidates for the country's second democratic presidential election.
Guinea's opposition on Thursday called for the vote to be postponed until later in October to allow the electoral commission to correct "anomalies and irregularities in the electoral roll".