Human Rights Watch said its study also found that the armed forces tried to hide evidence of their crimes by seizing bodies from the stadium, where the rally was conducted, and the city's morgues to bury them in mass graves.
"There is no way the government can continue to imply the deaths were somehow accidental," HRW's Africa director Georgette Gagnon said in a release. "This was clearly a premeditated attempt to silence opposition voices."
The human rights group said its 10-day research confirms that the death toll from the massacre was much higher than the government's official toll of 57 dead, more likely to be about 150 to 200 dead.
After security forces surrounded and blocked the stadium, they "stormed in and fired at protesters in cold blood until they ran out of bullets," Gagnon said. "They carried out grisly gang rapes and murders of women in full sight of the commanders. That's no accident."
In 2008, a group of Guinean military officers calling themselves the National Council for Democracy and Development seized power hours after the death of President Lansana Conte. The CNDD is led by self-proclaimed president, Capt. Moussa Dadis Camara. The rally was called to protest Camara's anticipated presidential run.
Among other things, HRW repeated its call for an the international commission of inquiry into the violence as proposed by the Economic Community of West African States, singling out several leaders, for investigation on criminal charges.