The military government, which seized power in a coup following the death of the president last December, has insisted that only 57 people were killed and released their bodies shortly after the incident.
It said that the majority of them had been killed in a stampede inside the stadium in the capital Conakry where the demonstration was being held.
However, a victim's association has compiled a list of 108 other victims who were known to have been at the stadium and whose bodies have never been recovered by their families.
Human Rights Watch estimated the number of victims at between 150 and 200 people. It said that hospital records showed that more than 1,400 people were admitted for treatment.
"What I want is for people to help us so that we can know where they buried my husband. Even if it's just bones, I need to find him"
The organisation also said that during an investigation into the massacre it had found "strong evidence ... that the military engaged in a systematic effort to hide the evidence of their crimes and misrepresent the number killed".
Among the evidence were witness accounts of the removal of bodies from city morgues and a military camp for burial elsewhere, in some cases in mass graves, the organisation said.
"We know that bodies were removed by the military from the stadium and from the morgues. They had to have been taken someplace, so it's likely that they ended up in mass graves," Corinne Dufka, a senior HRW researcher, said.
"They owe it to the family members of those who disappeared to reveal what really happened."
Human Rights Watch said it interviewed about 240 victims, witnesses, military, medical staff, diplomats and others to compile a picture of what happened at the stadium.
The report said that several hundred troops from the presidential guard along with gendarmes on the anti-drug and organised crime squad, some anti-riot police and dozens of militiamen in civilian dress entered the stadium and took posts on the edge and near exits.
'Spraying the crowd'
Tear gas was fired, causing panic, and security forces then stormed the stadium, firing into the crowd. The report said numerous witnesses spoke of "spraying the crowd from left to right".
"Witnesses described how the panicked demonstrators were gunned down as they attempted to scale the stadium wall; shot point blank after being caught hiding in tunnels, bathrooms, and under seats; and mowed down after being baited by disingenuous soldiers offering safe passage," the report said.
Khadiatou Barry told The Associated Press she had still not received her husband's body despite it being seen mobile phone footage taken inside the stadium.
"What I want is for people to help us so that we can know where they buried my husband. Even if it's just bones, I need to find him," she said.
The military government has denied responsibility for the massacre, saying it was the actions of a small number of rogue military personnel.
The European Union imposed sanctions on Guinea after the massacre and the United Nations sent a team to investigate.
Their report is due in the coming days and is expected to recommend prosecution for members of the government for committing crimes against humanity.