The US Embassy in Libya said it shared the UN mission's concerns over violence relating to the December 24 election, which is part of a peace process meant to end a decade of turmoil but has stirred fears of renewed conflict.
The elections commission disqualified Saif Al Islam Qadddafi, son of the late Libyan dictator, and 24 others on Wednesday. Rival camps are disputing the election rules and eligibility of candidates threatening to derail the vote.
Qadddafi's lawyer, Khaled Al Zaidi, said in a video that armed men had raided the court in the southern city of Sebha, one of only three registration centres, and stopped him entering to lodge his client's appeal.
The Justice Ministry in Tripoli said an armed group had forced everyone to leave the court building.
No faction claimed responsibility for the attack.
In a statement, the US Embassy said: "Attacks against judicial or election facilities or judicial or elections personnel are not only criminal acts, punishable under Libyan law, but also undermine Libyans’ right to participate in the political process."
Saif Al Islam Qadddafi's candidacy was rejected on the basis of a 2015 conviction in absentia by a Tripoli court for war crimes committed during the fighting that ousted his father, Muammar Qadddafi, in 2011.
He has spent the past decade in the mountain town of Zintan, where his captors took him after he was seized trying to flee Libya during the uprising.