The unrest on Monday occurred near a stadium in Conakry, the capital, where demonstrators had massed to protest against the expected decision of Moussa Dadis Camara, the country's military leader, to stand in forthcoming elections.
At least 58 people are reported to have been killed after security forces opened fire on opposition protesters in Guinea, a human rights group has said.
"At one hospital alone, we have counted 58 bodies," Thierno Maadjou Sow, the president of the Guinean Human Rights Organisation, told the Reuters news agency.
"It seems there are many more corpses in [the other hospital]."
A doctor at the Donka university medical centre appeared to confirm the toll given by the human rights group telling the AFP news agency: We have counted 52 bodies and six more have just come in."
Other medical officials were quoted as saying that "dozens of bodies" had been taken from another hospital by the military.
'Police state'
The protesters, who had gathered outside the stadium, carried placards reading "No to Dadis" and "Down with the army in power" before police stopped them from gaining entry.
Al Hassan Silah, a local journalist who visited the stadium shortly after the security forces had opened fire, said he and other journalists had seen bodies lying inside and outside the stadium.
"People did not wish for a military regime in Guinea because for almost 25 years Guinea was led by a military man" Lansine Kaba,
Guinean scholar
"People who had gone to attend the protests have fled for dear life," Silah told Al Jazeera."But 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."The military government, which took power last December, plans to hold elections in January 2010.Lansine Kaba, a Guinean scholar at the Carnegie Mellon University in Qatar, said: "Many people don't want the president to stand for election. He promised when they had the coup that he would not stand for elections.
"People did not wish for a military regime in Guinea because for almost 25 years Guinea was led by a military man," he told Al Jazeera
The government had banned Monday's demonstration, but a coalition of political parties, trade unions and civic organisations vowed that the event would go ahead.
Leaders arrested
Police arrested at least 30 people who were driven away in lorries, witnesses said.
Cellou Dalein Diallo, the leader of the opposition Union of Democratic Forces of Guinea (UFDG), and several other politicians were reportedly among those detained.
Mouctar Bah, a correspondent for the AFP news agency who also works for Radio France Internationale (RFI), was manhandled by uniformed men who took his microphone and recorder before shoving him to the ground.
Speaking on national television on Sunday, Frederic Kolie, the interior minister, declared that "all demonstrations on national territory are prohibited until the national holiday on October 2".
News of the ban came a day after Camara - in his first visit outside the capital since he took power - went to Guinea's second city and opposition stronghold Labe.
Camara seized power in the francophone West African nation after leading a bloodless coup within hours of the death of Guinea's president Lansana Conte, who had been in power since 1984.
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.