At least half a million Guineans joined a massive opposition rally in Conkary on Tuesday to denounce alleged economic mismanagement and corruption by the government of President Alpha Conde.
Union of Democratic Forces of Guinea (UFDG) leader Cellou Dalein Diallo said 700,000 people had joined a 15-kilometre (9.3-mile) procession from the suburbs to the 28 Septembre stadium in the west African nation's capital.
Security forces said the numbers were closer to 500,000.
Supporters of several opposition parties gathered for the event, shouting "Alpha resign, Alpha that's enough, students unemployed, we want jobs," brandishing placards reading "death to dictatorship."
Diallo hailed the huge turnout and said it proved the "rejection of dictatorship and poor governance of Mr Alpha Conde."
Diallo blamed Conde's government specifically for mishandling the massive Simandou iron ore project in the south of the country managed by British-Australian firm Rio Tinto, which he said meant Guineas could miss out on "decent jobs".
The government said in July the challenges of getting the project off the ground during a global iron ore glut were considerable but it would "do everything" to ensure it went on-stream.
Guinea's constitutional court in November 2015 formally confirmed Conde's re-election, dismissing opposition claims of vote rigging and fraud.
It was only the second democratic presidential poll since Guinea gained independence from France in 1958.
In addition to focusing on the economy, rights campaigners have urged Conde to use his second term to intensify the fight against impunity, strengthen the judiciary and promote equal respect for the rights of all Guineans.
Despite being rich in minerals, the majority of Guinea's population lives in poverty and survives on less than one euro ($1.08) per day, according to the UN.