But the prosecutor, Luis Moreno-Ocampo, said there were also allegations that Africans from other nations had been attacked in rebel territory because they were thought to be mercenaries.
Moreno-Ocampo is to brief the UN Security Council on Wednesday on his investigation which was ordered by the council when it passed resolutions in February allowing military action to protect civilians and sanctions against Gaddafi.
"We have been collecting evidence about crimes against humanity committed in Libya," Moreno-Ocampo told AFP. He said he believed he has enough evidence for cases to be launched.
"We have security forces shooting civilians at demonstrations and evidence of security forces arresting people in different cities, including Tripoli, even today, because they think these people are not loyal."
He said there had been a "massive arrest of people who are not considered loyal.
"In Libya it is illegal to challenge the story. They are arresting people who talk to journalists, and then they are torturing them."
The prosecutor said he had informed the UN council that prosecutors would present evidence in coming weeks for a judge to decide whether cases should be pursued.
Serious allegations of rape
Between one and five cases will be presented, Moreno-Ocampo said. "We are still defining who are the most responsible according to the evidence." He would not give the names of those suspected.
"I will inform the Security Council that we are ready, we will present the case. They can plan in advance how the warrants will be executed."
Moreno-Ocampo said there were "serious allegations of rapes" but he was not sure there was enough evidence to pursue a case now. There were also allegations of the use cluster bombs.
He added that in the rebel capital of Benghazi, there had been "cases of people attacking Africans because they were thinking these were mercenaries." This investigation is also being pursued.
"We are impartial. The international court is intervening in Libya because we have to protect civilians."
Ocampo announced a Libya crimes against humanity probe on March 3, which would look at the role of Muammar Gaddafi and three of his sons.
He was also said to be targeting four other top officials, including former foreign minister Musa Kusa and former prime minister Abu Zeyd Omar Dorda, director-general of the Libyan External Security Organisation.