The list of main suspects has not been made public, but it is known to include several ministers and members of parliament.
"I consider the crimes committed ... crimes against humanity, the gravity is there. Therefore I should proceed," Moreno-Ocampo said on Thursday.
He made the statements at a joint news conference with Mwai Kibaki, the Kenyan president, and Raila Odinga, the prime minister, in Nairobi.
Kibaki and Odinga were rivals in the violence-marred December 2007 election that claimed the lives of 1,500 people.
The bloodletting started after the electoral commission declared Kibaki the winner, and Odinga cried foul.
Al Jazeera's Andrew Simmons, reporting from Nairobi, said there is no doubting that Moreno-Ocampo is determined to seek justice for the perpetrators of the violence.
But Simmons said the day's developments represented a setback, because the ICC official did not achieve his "preferred option".
"There were two options: one was that the Kenyan leadership would refer themselves to the ICC. That did not happen," our correspondent said.
"While the Kenyan leadership said it would co-operate, it is now up to Mr Moreno-Ocampo to do it on his own. He will go to a pre-trial hearing in The Hague in December and get an investigation ordered."
Amos Wako, Kenya's attorney-general, admitted on Wednesday that he was the first official to have his US visa revoked as part of sanctions imposed by Washington on officials accused of involvement in the post-election violence.
Kenya had promised to deal with the suspected leaders of the clashes, but attempts to start the process failed and many Kenyans are sceptical those responsible will be arrested and charged.
Kofi Annan, the former UN chief, who acted as a mediator during the political unrest, has said that unless those responsible for the killings are penalised, there is a risk violence will erupt again during the next presidential election in 2012.
"We welcome Ocampo, he should come and take immediate action," Musa Muthumbi, chairman of a camp for displaced Kenyans in the Rift Valley town of Nakuru, told the Reuters news agency.
"We have had this problem, not once, not twice, but every election year since 1992. They have been doing this to us and getting off scot-free.
"This man should just come in and whoever is guilty should be taken to The Hague."