A blast ripped through a courthouse in the eastern city of Benghazi on Friday causing serious damage to the building but no casualties, Libyan security sources said.
The explosion punched a hole three metres (10 feet) wide in the walls of the courthouse on Meidan al-Shajara, a public square flanked by several government offices and the National Oil Corp, an AFP journalist said. It also damaged a court room.
Security sources told AFP "unknown assailants booby-trapped the building" and set off the explosion at "exactly 5 am (0300 GMT)."
Witnesses and residents told AFP the blast was very intense.
"It left residents in the area and patients being treated in a nearby hospital completely shaken up," one said.
Government spokesman Mohammed al-Harizi told AFP no one was wounded and that investigations were under way to determine the identity and motives of the assailants.
He said investigators had found graffiti at the scene expressing support for former leader Moamer Kadhafi, who was captured and killed by rebels in October.
"They found some slogans related to Kadhafi's regime," Harizi said, without elaborating.
The blast came just hours after the arrival of the ruling National Transitional Council, which usually meets in Benghazi during the final week of every month.
Late Thursday, clashes sparked by a prison revolt in Benghazi, cradle of the revolt that toppled Kadhafi, left one person dead and at least four wounded.
Security sources blamed the prison-break attempt on radical Islamists.
The Mediterranean city has been hit by sporadic acts of violence this year, including an attack on government buildings, the desecration of a World War II cemetery, clashes at political rallies and the failed bombing of a UN convoy.
Libya is also facing a leadership crisis with key members of the ruling National Transitional Council pushing for a no confidence vote against the government of interim Prime Minister Abdel Rahim al-Kib.
Harizi said "no decisions" had been reached on that topic.