THE HAGUE  The International Criminal Court has started a formal investigation into possible crimes against humanity in Libya that will focus on the role of the country’s leader, Col. Muammar el-Qaddafi, several of his sons and members of his inner circle, the chief prosecutor said Thursday.

Luis Moreno-Ocampo, the prosecutor, said his office had received confirmation that Libyan security forces had fired on peaceful demonstrators, killing hundreds, and that many had been illegally detained in episodes involving at least nine different towns since Feb. 15.

He also said in an interview that judging by the information he had received, many more insiders from the Libyan government had defected than was publicly known. “The system appears to be breaking down,” he said, adding that reports about recent events and the inner workings of the government had come from multiple sources. “We cannot name names to protect the families,” he said.

In a unanimous vote on Saturday, the United Nations Security Council instructed the international prosecutor to investigate the violence in Libya, and it approved a series of other measures including imposing an arms embargo on the country, banning international travel for 16 Libyan leaders and freezing the assets of Colonel Qaddafi and members of his family.