At least three protesters were killed in the Guinean capital, Conakry, when security forces broke up an opposition demonstration.
Police used tear gas and batons against the stone-throwing protesters.
Dozens of police vehicles and paramilitary forces prevented opposition activists from reaching a stadium.
The clash took place in the run-up to parliamentary elections, due to be held in December.
Guineans voted in presidential elections last November, two years after the military seized power.
Parliamentary polls should have been held within six months but have now been fixed for 29 December by the authorities and the Independent National Electoral Commission (CENI).
Opposition leaders say they fear the elections will be a sham.
Leading opposition figure Cellou Dalein Diallo, who narrowly lost in last year's poll, has accused President Alpha Conde of installing a close ally as CENI head and of trying to tamper with voter rolls.
A meeting on Monday between Prime Minister Mohamed Said Fofana and opposition leaders brought no resolution and opposition leaders said they had decided to go ahead with the march despite a ban.
This was to have been the first major demonstration since last year's election and many shops and petrol stations were closed for fear that violence could break out, said reports.
Police and paramilitary force vehicles blocked access to the September 28 stadium, where the rally was to be held, and other parts of the capital.
"There were youths on the hilltops throwing rocks. The security forces fired back with tear gas, which triggered general panic," Souleymane Bah, a resident of the Conakry suburb of Bambeto, told Reuters news agency.
The crackdown on the protest was "regrettable", opposition figure Sidya Toure told AFP.
The incident took place on the eve of the second anniversary of a massacre in the stadium when soldiers loyal to the government shot dead more than 150 unarmed protesters.
The former colonial power France has called for "calm and restraint", AFP reported.