The International Criminal Court is to investigate war crimes allegedly committed in the Central African Republic (CAR) in 2002 and 2003
The Hague court's prosecutor said his investigation would mainly focus on the large number of alleged rapes.
The period covers the aftermath of a failed coup by current President Francois Bozize against the government of former leader Ange-Felix Patasse.
The CAR's Supreme Court had referred the matter to the ICC in 2004.
The CAR court said it did not have the ability to prosecute such cases.
The ICC prosecutor, Luis Moreno Ocampo, said: "The information we have now suggests that the rape of civilians was committed in numbers that cannot be ignored under international law."
His office has taken testimony from hundreds of alleged rape victims, some of whom said they were assaulted in public or in the presence of family members.
Mr Moreno Ocampo said: "These victims are calling for justice."
Last year, the CAR court referred Mr Patasse and the Vice-President of the Democratic Republic of Congo, Jean-Pierre Bemba, to the ICC on charges of rape and murder.
A French policeman and two aides of Mr Patasse were also referred. None has been arrested.
The alleged crimes took place after Mr Bozize instigated a coup in October 2002.
The Congolese Liberation Movement, under Mr Bemba, came to help Mr Patasse.
Mr Bozize finally came to power in the landlocked country of 3.9 million people in 2003.
Mr Moreno Ocampo said no particular suspect was being targeted in the investigation.
But he said: "We have a duty to show that massive crimes cannot be committed with impunity."
International Criminal Court opens probe into Central African Republic