"The military is going into districts, looting goods and raping women," Mamadi Kaba, the head of the Guinean branch of the African Encounter for the Defence of Human Rights (RADDHO), told AFP.
"We have similar reports from several sources, including police sources and some close to the military," said Mr Kaba, from his office in Dakar, Senegal.
The interior ministry source admitted that some soldiers had fired live rounds into the crowd, but said that only four people had died from gunshot wounds. The others, the ministry said, were trampled to death.
The opposition has accused the army of taking away some bodies to hide the scale of the violence.
Capt Camara denied knowledge of sexual assaults, but admitted that some of his security forces had lost control.
He said he was waiting to hear exactly how many people had died.
"Frankly, it saddens me immensely. Frankly, it is very regrettable," he told French radio.
Capt Camara said he had not yet decided whether to run for the presidency and was unsure what the correct move would be.
There has been worldwide condemnation of the violence.
UN Secretary General Ban Ki-moon urged the Guinean authorities to exercise maximum restraint, while the West African regional body Ecowas is reported to be pursuing sanctions against the military regime.
The African Union has expressed grave concern over the latest violence, condemning the "indiscriminate firing on unarmed civilians".
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