The 16-year-old was killed in front of his home in an opposition stronghold area of the capital Conakry as security forces
chased demonstrators returning from an opposition rally, the source told AFP, speaking on condition of anonymity.
"The crackdown led to five people receiving gunshot wounds who were cared for in various health centres and clinics throughout the city," the source added.
The death was confirmed by opposition leader Cellou Dalein Diallo, who visited the morgue.
"This is proof of the barbarity of the police who chased the opposition activists towards their family homes to kill them," said Diallo, who had called the protest.
Thousands had gathered in the city's suburbs before beginning a March surrounded by police to protest against a "unilateral" government decision to hold legislative elections on June 30.
"We want free and fair elections, we want Guinea to be a truly democratic country where life is good," said former prime minister Sidya Toure.
The poll, postponed several times in the past two years, has become a hit button issue in the west African nation as it tries to move on from decades of dictatorship, coups and political violence.
The most recent date of May 12 was abandoned after violent opposition protests broke out in February, leaving nine dead and more than 240 injured, and 25 people were injured when security forces and protesters clashed again last week.
The last legislative elections in the country were held in 2002 under then president Lansana Conte who ruled the former French colony for 24 years until his death in December 2008, prompting a disastrous coup marked by extreme police brutality.
Government spokesman Damantang Albert Camara, informed of the death by AFP, described the shooting as "regrettable".
In a statement issued earlier he had reported "four injured, two protesters and two police officers".
"The government regrets and condemns the turn that the protest, announced as peaceful and authorised by the administration, has taken in Conakry, descending into violence that no one would want," the statement added.