Photo: AFP/GETTY
Opposition parties had organised the protest in the capital's main football stadium, which drew some 50,000 people. Demonstrators chanted "We want true democracy."
But soldiers from the presidential guard entered the stadium and fired into the crowd, reportedly using tear gas, live ammunition and baton charges against the demonstrators.
The number killed in the attack remains unclear, but witnesses reported seeing dozens of bodies.
A doctor, speaking on condition of anonymity, told AFP that 58 people had been brought to a Conakry morgue on Monday.
"We have counted 52 bodies and six more have just come in," said the doctor at the Donka university medical centre.
But a member of the Red Cross told AFP that military commanders had issued instructions for all bodies to be taken to the Alpha Yaya Diallo military camp, rather than to morgues.
Elsewhere, at Ignace Deen hospital, a medical source told AFP that an army truck had come by to pick up "dozens of bodies" to be taken to "an unknown destination".
Tensions have risen amid rumours that military leader Capt Moussa "Dadis" Camara may run in presidential elections scheduled for Jan 31.
Capt Camara came to power in a coup last December, hours after longtime dictator Lansana Conte died. He initially said he would not run in the election, but has recently said he has the right to run if he chooses.
Witnesses and medical officials say at least 50 people have been killed in Conakry, the capital of Guinea, in clashes between security forces and opposition activists who defied a government ban on protests.

Conakry, the capital of Guinea (2007) 
Witnesses say Guinea's security forces opened fire on demonstrators who had gathered in a large stadium to protest against the possible presidential candidacy of Captain Moussa Dadis Camara, the country's military leader. They also say police charged protesters with batons, used tear gas, and detained several opposition leaders. 


"We have counted 52 bodies and six more have just come in," the city's university medical centre's doctor said.

On Sunday, Captain Camara's government said all protests would be prohibited until national independence celebrations are held on October 2. 

"All demonstrations on national territory are prohibited until the national holiday on October 2," said Frederic Kolie, the interior minister, in a televised address on Sunday.

However, opposition activists decided to proceed with the demonstration they had planned for Monday. Some carried signs that read "No to Dadis", and others chanted "We want true democracy." Some of the demonstrators set furniture on fire as they marched from the outskirts of the capital into the city. 

"... Conakry can be rightly described as a police state ... All across town, military people at checkpoints are searching people and people have been forced back into their houses," said Al Hassan Silah, a local journalist, to the Al Jazeera news agency.

Moussa Dadis Camara.
Captain Camara took power in a coup last December, following the death of Guinea's longtime President Lansana Conte.

When he took power, Captain Camara said no one in his ruling council would run for public office. However, the council has since said its members are eligible to be candidates, and Captain Camara has suggested he may run for president in elections, which are scheduled for January 2010. 

The African Union said it may impose sanctions on Guinea if Captain Camara decides to run for president. It said it was concerned about what it called a "deteriorating situation" in the country and the consequences of not returning to constitutional order.

Opponents have also accused Mr Camara and his ruling council of human rights abuses and limiting freedom of speech. 

Supporters of Guinea's military council are calling the threatened sanctions unfair.