The government planned to crush protests by killing civilians even before the uprising in Libya broke out, the International Criminal Court says.
The chief prosecutor said the plans had been a reaction to street protests that led to the fall of the Tunisian regime.
Luis Moreno Ocampo is expected to request arrest warrants for Col Muammar Gaddafi, his sons and close aides.
Nato has vowed to do all it can to protect civilians after rebels accused it of failing to protect Misrata city.
"Misrata is our number one priority," Nato deputy spokeswoman Carmen Romero told the AFP news agency on Wednesday.
She added that alliance warplanes had hit Col Gaddafi's military assets around Libya's third largest city on Monday.
"We have a clear mandate and we will do everything to protect the civilians of Misrata," Ms Romero said, referring to the UN resolution that permits "all necessary measures" to defend Libya's population.
On Tuesday, rebel commander Gen Abdul Fattah Younis complained that bureaucracy was causing Nato to take hours to respond to calls for air strikes.
"[Nato] is letting the people of Misrata die every day," he told reporters in the de facto rebel capital Benghazi. "If Nato should wait another week, there will be no more Misrata."
In The Hague on Tuesday, Mr Moreno-Ocampo said: "We have evidence that after the Tunisia and Egypt conflicts in January, people in the regime were planning how to control demonstrations inside Libya.
"The planning at the beginning was to use tear gas and [if that failed to work]... shooting," he told Reuters.
Doctors said last week that at least 200 people had been killed there since the uprising began on 17 February - a figure likely to have risen in recent days.
Nato says international air strikes have reduced Col Gaddafi's military capabilities by nearly a third, but his forces have deliberately moved weaponry into civilian areas to hamper air strikes.
A Nato-led coalition mandated by the UN to protect civilians is enforcing a no-fly zone and attacking ground targets. It accuses Gaddafi forces of sheltering weapons in civilian areas.
Rebel forces in the east of the country have retreated after heavy bombardment from Col Gaddafi's army, the BBC's Wyre Davies says from the front line, near the town of Ajdabiya.
In the past 24 hours, heavy and accurate shelling from Gaddafi troops has pushed the disorganised and poorly-equipped rebel army back to the fringes of the town, our correspondent adds.
It is a clear indication that without the protective cover of Nato air strikes, they would find it almost impossible to take the next town of Brega, never mind marching on Tripoli and removing Col Gaddafi from power, he adds.
In a boost to the rebels on Tuesday, a tanker arrived at the eastern port of Tobruk to pick up the first consignment of oil to be exported from the rebel-held region.
The Greek-owned ship is capable of carrying more than $100m (£61m) worth of oil.
Since the revolt against Col Gaddafi's rule began nearly two months ago, exports of Libya's main commodity have collapsed, driving up the price of oil to a two-and-a-half year high.
Also on Tuesday, the Gaddafi government made senior diplomat Abdelati Obeidi its new foreign minister, replacing Moussa Koussa, who sought refuge on the UK last week.