Guinea's opposition leader Cellou Dalein Diallo declared on Monday declared himself winner of Sunday's vote, defeating Alpha Conde, the incumbent president.
Speaking at a press conference in the capital Conakry a day after the hotly contested election, Diallo said he had emerged "victorious" despite "anomalies which marred the ballot".
"I invite all my fellow citizens who love peace and justice to stay vigilant and committed to defend this democratic victory," the 68-year-old said, dressed in a sky-blue robe, from his party headquarters.
Outside the building, supporters erupted in joy and chanted "Cellou, president". Elsewhere in the city, security forces fired tear gas canisters at crowds assembling in support of Diallo.
Bakary Mansare, the vice-president of Guinea's electoral authority, told AFP that Diallo's self-proclaimed victory was "null and void".
"It is not up to a candidate or a person to proclaim himself the winner outside the bodies defined by the law," he said.
Conde's RPG party also said in a statement Monday that it condemned "with the utmost firmness the irresponsible and dangerous declaration" by Diallo. It called for its activists to remain calm.
Diallo's announcement sets the stage for a showdown with the government, which insists that Sunday's vote was fair and that the official electoral authority must declare the results.
Signs of a looming electoral dispute began to appear during the vote, however, when Diallo told reporters that Conde could "cheat" his way to power.
Ousmane Gaoual Diallo, a cadre in Diallo's UFDG party, said that results at individual polling states were public, enabling the party's own observers to conduct a count.
"If we are the winners, we will defend our victory," he said. "We won't wait."
Earlier on Monday, Guinea's government said in a statement that the opposition "clearly intended to create chaos and to call into question the real results that will come out of the ballot box".
Much of the tension in Guinea relates to President Conde's controversial bid for a third term.
He pushed through a new constitution in March which he argued would modernise the country. But it also allowed him to bypass a two-term limit for presidents, provoking mass protests.
Prime Minister Kassory Fofana said that the opposition publishing results ahead of the official results was tantamount to pouring "oil on the fire".
Guinea's acrimonious political campaign saw Conde and Diallo trade insults, and was marked by violent incidents in some parts of the west African 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.