Libya's government and the UN expressed concern Monday over an intimidation campaign that has shut the court where the son of slain dictator Moamer Kadhafi is appealing his rejected presidential bid.

The elections on December 24 come as Libya seeks to turn the page on a decade of violence since a NATO-backed uprising that toppled and killed Kadhafi in 2011.

Last Wednesday, Libya's electoral commission announced the rejection of the candidacy of his son Seif al-Islam Kadhafi, who is wanted by the International Criminal Court for alleged war crimes.

Unsuccessful applicants were given 48 hours to appeal in court.

But on Thursday morning, a "group of outlaws" launched an "odious" attack on the court in the southern city of Sebha, forcing it to shut hours before Seif al-Islam had been due to appeal, the government in Tripoli said.

Similar incidents have been reported since, with footage broadcast on local media showing gunmen -- presented as belonging to the eastern-based strongman and presidential candidate Khalifa Haftar -- blocking access to the court.

The Libyan government said in a statement late Monday that it was "following with great concern the tensions around the court in Sebha, which threaten the legitimacy of the judicial institution".

"Threatening the lives of judges or trying to influence their work risks plunging Sebha back into civil war," it added.

The United Nations Libya mission UNSMIL also said in a statement that it was "following with great concern the continued closure of Sebha Court of Appeal" where it said "judges were physically prevented from conducting their legally mandated duties, directly impeding the electoral process".

"UNSMIL is also alarmed by increasing reports of intimidation and threats against judges and judicial employees, particularly those dealing with electoral-related complaints, as well as against candidates, in a number of locations in Libya."

Dozens of people, including supporters of Seif al-Islam, protested in Sebha on Monday to denounce an "attack on the work of justice".

Seif al-Islam was among 25 of the 98 hopefuls whose presidential bids were turned down.