Thousands of distraught relatives identified bodies after the crackdown
A deadly crackdown on protesters in Guinea in September was "premeditated and pre-planned at the highest level", Human Rights Watch has told the BBC.
Soldiers deployed at the sports stadium where protesters had gathered blocked the exits before systematically killing and raping protesters, the group says.
Activists say 157 people were killed but officials say far fewer died and claim most were trampled to death.
The EU is the latest international body to put sanctions on the ruling junta.
The bloc put an arms embargo on the country and imposed sanctions against its military leaders.
EU officials have joined pressure groups, including US-based Human Rights Watch (HRW), in calling for members of the junta to be tried for human rights violations.
HRW's Corinne Dufka, who led the group's research, told the BBC's Network Africa programme that the soldiers deployed at the Conakry stadium on 28 September were mainly from the elite presidential guard.
She said they blocked key exits before moving through the crowd, firing at protesters and raping women.
"The fact that the rape happened, that it broke out at the same time, suggests that there was some kind of at least permission given," she said.
The group spoke to dozens of rape victims - many of them telling of gang rapes and saying that the troops had referred to their ethnicity as they were carrying out the attacks.
It is exactly a month since the crackdown, and unions and activists have called for a nationwide strike to remember the victims.
"We, the women of Guinea, are wounded in our souls and humiliated by the events of 28 September," rights activist Diaraye Haidara told the Associated Press news agency.
"A day of remembrance will certainly make us feel better but it will never heal our open wounds from these rapes."
The rally was called to protest at reports that junta leader Captain Moussa Dadis Camara was planning to stand in presidential elections next year.
After seizing power in December 2008 following the death of long-time leader Lansana Conte, he pledged not to contest elections but has since made a series of contradictory statements about his intentions.
Following the stadium killings, he has described himself as a "hostage" - both to the Guinean people and to the "unstructured" army.