Moreno-Ocampo said he has a list of 20 possible suspects made up of political and business leaders and that he will prosecute "those most responsible".
'Crimes against humanity'
A day earlier, the ICC authorised the prosecutor to investigate the post-election unrest, in which about 1,200 people died.
The ICC said in a statement issued on Wednesday that "the information available provides a reasonable basis to believe that crimes against humanity have been committed on Kenyan territory".
Weeks of violence followed the 2007 general election, with about 400,000 people fleeing their homes. Hundreds were killed in tribal clashes and police were accused of brutality.
Al Jazeera's Andrew Simmons said that outside Moreno-Ocampo's list of 20 top suspects, there were many others involved in the unrest.
"Beyond those 20, there are hundreds, maybe thousands, of killers, who haven't been brought to justice."
He also said that the witnesses Morano-Ocampo vowed to protect were just a small number of the victims.
"There are countless others out there, in hiding, worried about their lives ... Most human rights campaigners believe there's no such thing as independent justice in Kenya and there's no such thing as independent protection for those who are prepared to risk their lives to give evidence in court."
Kenya's failure to set up a tribunal to try suspects of the unrest prompted Kofi Annan, the former UN secretary-general, to hand in the names of important suspects to the ICC.
Annan has welcomed the investigation decision, describing its start as an "important day for justice."
"Justice for the victims suddenly looks brighter. I urge all concerned to fully co-operate with the ICC," he said in a statement on Wednesday.
Mutula Kilonzo, the Kenyan justice minister, also "welcomed" the investigation.
He also said that victims who give evidence are already protected under Kenyan law.
"We have already on the ground a law on witness protection ... and right now there is a draft in parliament seeking to amend it to meet international practices on witness protection," he told Al Jazeera.
"The country has not only local but international obligations that the witnesses are protected. The world is watching, we are well aware of that."
Kenya has agreed to provide the court with confidential information about incidents during the unrest.
Mark Ellis, the executive director at the International Bar Association, praised the ICC's decision on Wednesday.
"I think it is a very important step for Kenya and the international community," he told Al Jazeera.
"The ICC provided an opportunity for Kenya to undertake their own investigation and prosecution, but they were not able to do that.
"It is morally and legally right for the International Criminal Court now to undertake this first step and that is the investigation into the crimes that were committed in Kenya."
Ellis said Kenyans cannot enjoy "sustainable peace and certainly not reconciliation" unless those who have committed the atrocities are brought to justice.
The ICC is the world's first permanent court mandated to bring to justice perpetrators of war crimes, crimes against humanity, and genocide when national courts are unable or unwilling to do so.
Currently, 110 states are parties to the ICC.