Moreno-Ocampo's office reviewed more than 1,200 documents and 50 interviews
The International Criminal Court's prosecutor is seeking the arrest of Libyan leader Col Muammar Gaddafi and two others for crimes against humanity.
Luis Moreno-Ocampo said Col Gaddafi, his son Seif al-Islam, and intelligence chief Abdullah al-Sanusi bore the greatest responsibility for "widespread and systematic attacks" on civilians, reports the BBC.
ICC judges must still decide whether or not to issue warrants for their arrest.
The Libyan government has already said it will ignore the announcement.
Deputy Foreign Minister Khalid Kaim said the court was a "baby of the European Union designed for African politicians and leaders" and its practices were "questionable".
Libyadid not recognise its jurisdiction, like most African countries and the United States, and would ignore any announcement, he added.
Earlier, Moreno-Ocampo's office said that after reviewing more than 1,200 documents and 50 interviews with key insiders and eyewitnesses, he would request later on Monday that the ICC's Pre-Trial Chamber issue three arrest warrants.
The men are suspected of committing two categories of crimes against humanity - murder and persecution - under the Rome Statute which established the court.
The charges cover the days following the start of anti-government protests on February 15.
"The evidence shows that Libyan security forces conducted widespread and systematic attacks against the civilian population and led to the identification of those who bear the greatest criminal responsibility for such crimes," a statement from the prosecutor's office said.
"Additionally, there is relevant information on alleged commission of war crimes once the situation developed into an armed conflict. The office will evaluate these crimes with the same standards, in particular allegations of rape and attacks against sub-Saharan Africans wrongly perceived to be mercenaries."
An inquiry set up by the UN Human Rights Council is expected to submit its report on the alleged war crimes to the UN Security Council on June 7.
The application for warrants is expected to focus on the initial clampdown against protesters by Col Gaddafi's government. Between 500 and 700 people are believed to have been killed in February alone.
Moreno-Ocampo said he was acting in accordance with UN Security Council resolution 1970, which referred the situation in Libya to the ICC, and stressed the need to hold to account those responsible for attacks on civilians.
The Pre-Trial Chamber's judges may decide to accept the prosecutor's application, reject it, or ask him for additional information.
If Col Gaddafi is named, it would only be the second time the ICC has sought a warrant for a sitting head of state. Sudanese President Omar al-Bashir has been indicted for crimes including genocide in Darfur.
Meanwhile, the Libyan government has condemned British calls for NATO to bomb a wider range of infrastructure targets to put pressure on Col Gaddafi.
A spokesman said the comments by the Chief of the Defence Staff, Gen Sir David Richards, was a threat "aimed at terrorising civilians".
Gen Richards told the Sunday Telegraph: "The vise is closing on Gaddafi, but we need to increase the pressure further through more intense military action".
"The military campaign to date has been a significant success for NATO and our Arab allies. But we need to do more. If we do not up the ante now there is a risk that the conflict could result in Gaddafi clinging to power."
Overnight, Libyan state television reported said NATO aircraft had bombed an oil terminal in the eastern port of Ras Lanuf.
The alleged strike came after rebel fighters said they had taken full control of the western city of Misrata and said the situation was now "static".