The opposition had called for a nationwide day of protest against a disputed election timetable after two weeks of violent clashes in the capital Conakry, even as the president ruled out any review.
A hospital source in Labe, the country's second-largest city and an opposition stronghold, said the victim "was beaten with batons by security forces".
The protester's body was carried to a regional hospital by fellow activists who cried "God is greatest", "death to the dictator" and "justice for all", witnesses said.
Conakry governor Soriba Sorel Camara made a statement on public radio ahead of today's protests, criticising sections of the opposition for having "chosen the street" to make their case.
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He accused activists of "acts of rare barbarity" including "stoning public and private vehicles, seriously wounding public order officials" and he urged the public to go about their normal business.
Clashes broke out at midday between demonstrators and police in the suburbs of Conakry, with at least five civilians sustaining minor injuries, witnesses told AFP.
"The opposition has once again defied the law and endangered the lives of our people," it said in a statement.
Youths gathered in large crowds on the main roads in Conakry, AFP correspondents witnessed, facing off against security forces who had deployed in huge numbers in the capital and throughout the country.
Demonstrators and police also clashed in Kindia, Guinea's third-largest city and the headquarters of the country's military, 130 kilometres east of Conakry. Local media said a policeman and a protester had been wounded.