The International Criminal Court is to investigate Libyan leader Muammar Gaddafi and some of his sons, for possible crimes against humanity.
The move follows the violent crackdown on anti-government protesters.
Prosecutor Luis Moreno-Ocampo said Gaddafi's security forces are alleged to have attacked "peaceful demonstrators" in several towns and cities across Libya since February 15.
He identified Gaddafi and several commanders and regime officials as having formal or de facto command over the forces that may have committed crimes.
Moreno-Ocampo vowed there would be "no impunity in Libya".
Armed with unusual authority from the UN Security Council, Moreno-Ocampo acted with unprecedented speed to launch an investigation, partly to warn Libyan officials against any continued slaughter of civilians.
He said the court was using the opportunity "to put them on notice: if forces under their command commit crimes, they could be criminally responsible."
He also warned that leaders of the Libyan opposition, who have seized weapons from the Libyan military, could be investigated if allegations were raised against them.
"No one has the authority to attack and massacre civilians," he said.
Moreno-Ocampo mentioned only Gaddafi by name, but identified seven people by their positions to be investigated.
They were the commander of the 32 battalion, the head of Gaddafi's personal security, the national security adviser, the director-general of the external security organisation, the spokesman of the regime, the head of the security forces and the minister of foreign affairs.
Gaddafi's son Khamis commands the elite 32nd battalion.
Gaddafi has denied using violence against demonstrators, whom he described as agents of al Qaida. International media have been unable to witness the worst of the reported incidents.
Contacting former Libyan officials and army officers, Moreno-Ocampo said he spent the last six days gathering information on the structure of authority in Libya to find those in control of the forces.
He also has appealed for video and photographs of any alleged atrocities.
"We are not saying who is responsible yet," he said. "Today is the start of the investigation."
He said it could take several months before he presents his case to judges and requests arrest warrants.
It is the second time the court, which was created in 2002 as the world's first permanent war crimes tribunal, has investigated a sitting head of state.
Sudan's Omar al-Bashir has been indicted on three counts of genocide for attacks in the western Darfur region, but Bashir has rejected the charges against him and refused to surrender to the court.
The UN-backed court has no power to execute its own arrest warrants, and must rely on national authorities to take suspects into custody and extradite them to The Hague.