The West African states's main opposition leader, Cellou Dalein Diallo, announced on Monday that he had won the hotly contested October 18 poll without waiting for the official results.
Tensions remained high in Guinea after a night of unrest sparked by an opposition claim to victory in an election against President Alpha Conde, who is seeking a controversial third term.
The opposition Union of Democratic Forces of Guinea said early on Tuesday that three youths, aged 13, 14, and 18, were killed Monday night when security forces suppressed the party's celebrations.
“While supporters of President Cellou Dalein Diallo were peacefully celebrating the victory of their candidate, defence and security forces violently suppressed this moment of jubilation,” the party said in a statement.
Diallo announced on Monday that he had won the hotly contested October 18 poll ... without waiting for the official results.
"I invite all my fellow citizens who love peace and justice ... to defend this democratic victory," he had declared.
But President Alpha Conde's camp rejected the self-proclaimed win, and joyous celebrations from opposition supporters in the capital Conakry quickly descended into violent clashes with security forces.
Diallo, 68, tweeted on Monday night that security forces had fired into a crowd and shot dead "three boys", leaving several wounded. He blamed Conde for the "crimes".
Guinea's government did not confirm the deaths.
'Irresponsible and dangerous'
The unrest follows months of protests against a potential third term for 82-year-old President Conde in the poor former French colony, during which dozens of people were killed.
Although polling day was mostly calm, Diallo's self-proclaimed election victory has set the stage for a showdown with Conde.
The government insists the vote was fair and that only the official electoral authority can declare the results, which are due within a week.
Conde's RPG party also called Diallo's move "irresponsible and dangerous" on Monday.
The international community is concerned too.
The United Nations, African Union, and the 15-nation West Africa bloc ECOWAS, called the premature declaration of results "regrettable", in a joint statement on Monday.
"This state of affairs is not conducive to preserving calm," the statement said.
Bakary Mansare, the vice-president of Guinea's electoral authority, told AFP that Diallo's self-proclaimed victory was "null and void".
Signs of a looming electoral dispute had already surfaced during the vote, when Diallo told reporters that Conde could "cheat" his way to power.
Twelve candidates are vying for the presidency, but Conde and Diallo are the frontrunners.
Guinea's acrimonious political campaign saw Conde and Diallo trade insults, and was marked by violent incidents in some parts of country.
But it also raised the spectre of ethnic strife, with Conde accused of exploiting divisions for electoral ends, a charge he denies.
Guinea's politics are mainly drawn along ethnic lines: the president's base is mostly from the ethnic Malinke community and Diallo's from the Fulani people.
A second round of voting, if needed, is scheduled for November 24.