Rioting broke out around 9:30am (0930 GMT) in several suburbs including Simbaya and Hamadallaye, and angry protests continued as at least eight youths were treated for gunshot wounds.
The opposition had called for demonstrations in all parts of the capital against the ongoing lack of security for which it blames the regime of President Alpha Conde.
“There were violent clashes between young protesters and police in Hamdallaye, where security forces shot three young protesters,” a hospital doctor said on condition of anonymity.
The medic told AFP two of the protesters sustained leg injuries while a third with a serious stomach wound was dragged several metres by police before being abandoned.
“He was quickly rescued by his comrades,” a policeman told AFP.
The gunfire continued after the wounded were taken to hospital as youths rallied in the streets, according to an AFP correspondent on the ground who witnessed panicked residents trying to get home.
Four other protesters were wounded in the northeastern suburb of Simbaya, witnesses and a police officer told AFP on condition of anonymity.
Guinea’s opposition boycotted parliament in March in protest over the timetable for a presidential election, accusing Conde of using the Ebola epidemic as an excuse to postpone voting.
Former prime ministers Cellou Dalein Diallo, Sidya Toure and Lansana Kouyate accused Conde of repeated rights violations and said he had “lost all legitimacy”.
They called on supporters to back several demands including a call to bring forward local elections due in March next year.