A Guinean rights group has said 157 people were killed and more than 1,200 others wounded when protesters rallied against plans by Captain Moussa Dadis Camara, the country's military ruler, to run for president next year.
The government put the death toll at 57. Camara has blamed uncontrollable elements within the Guinean army for the killings, saying he cannot be held responsible.
"From the information we have received, from the pictures I have seen, women were abused or otherwise brutalised on the pitch of Conakry's stadium, apparently by men in uniform," Fatou Bensouda, the ICC deputy prosecutor, said in a statement.
"This is appalling, unacceptable. It must never happen again. Those responsible must be held accountable."
Earlier this week, thousands of Guineans stayed indoors to mark the killings, bringing the capital, Conakry, to a standstill.
The strike also froze production in the country's mining operations.
If the court finds crimes were committed, it could launch a full-scale investigation and prosecute senior commanders. The initial phase of the investigation will likely take months.
Camara seized power in a coup nine months ago, hours after the death of President Lansana Conte.
The African Union has given Camara until mid-October to confirm he will not stand in presidential elections slated for January 31, warning of sanctions if he misses that deadline.
The September violence drew international condemnation, with Hillary Clinton, the US secretary of state, saying Camara should quit while France cut military co-operation with its former colony.
Karel De Gucht, the European Union's aid chief, said Camara should stand trial for a "crime against humanity".
The International Criminal Court has so far launched four full-scale investigations in the DR Congo, Uganda, Sudan's Darfur region and the Central African Republic.