The violence broke out after Raila Odinga, now prime minister in a power-sharing government, accused his rival for the presidency, Mwai Kibaki, the incumbent president, of winning the December 27, 2007 poll through widespread fraud.
Hundreds of thousands of people were forced from their homes as the violence raged.
Moreno-Ocampo has been conducting a preliminary investigation into the clashes since February last year and said during a visit to Kenya last month that he wished to pursue "those responsible".
Al Jazeera's Andrew Simmons, reporting from Nairobi, said: "Ocampo has made it quite clear that he is looking for the top people, the ringleaders, those who financed and those who incited violence.
"He has told Al Jazeera in an interview in Kenya two weeks ago that he was only talking about two or three people, which leaves a lot of others who are responsible for the violence who could possible could evade justice."
In July, Kofi Annan, the former UN secretary-general who helped broker the power-sharing deal between Kenya's leaders, sent the ICC a list of names of key suspects that is believed to include senior government officials.
This is the first time that the ICC prosecutor has sought to open an official investigation on his own initiative, one of three ways in which a case can come before the international court.
Other cases before the court had either been referred by countries that have signed up to the court's founding Rome Statute or by the UN Security Council, as in the case of the conflict in Darfur, Sudan.
'Court of last resort'
Elizabeth Evenson, counsel in the international justice programme of Human Rights Watch, said: "The ICC is a court of last resort, and when national authorities are unwilling to act, it is supposed to step in.
|Moreno-Ocampo, left, has been investigating the clashes since February last year [AFP]
"Today's announcement shows that the ICC prosecutor can and will act on his own in situations of serious crimes."
The Kenyan government has yet to act on the recommendation of its own inquiry that a special tribunal be set up to investigate the violence.
Moses Wetangula, the Kenyan foreign minister, said earlier this month that Nairobi would assist any ICC to investigation, but was committed to a "local solution".
Hassan Omar of the kenyan Human Rights Commission said that the passage of time and the Kenyan government's willingness to co-operate were the key challenges facing any investigation.
"Once the authority is granted by the International Criminal Court it will be important to see how the government will react," he told Al Jazeera from Nairobi
"The government has been quite anxious in terms of how to deal with the International Criminal Court and there has been widespread anticipation that the gobvernment might resist any direct intervention ... on the basis that they are handling the matter internally."