BERLIN (REUTERS) - Police searched courtrooms in at least seven German cities on Friday (Jan 11) after bomb threats and security alerts, with at least one emailed warning signed: "Nazi offensive", officials and media said.

Police said they temporarily evacuated or searched courts in the eastern city of Potsdam, the western cities of Wiesbaden and Saarbruecken, as well as in the central cities of Magdeburg and Erfurt and in the northern ports of Kiel and Hamburg.

German Justice Minister Katarina Barley condemned the bomb threats and a separate arson attack on the Federal Supreme Court in Leipzig on Dec 31 which was reportedly claimed by a radical leftist group.

"Those who threaten courts attack the independent judiciary and our democratic society," Barley said.

She said it was imperative to investigate the cases carefully and hold the perpetrators accountable.

None of the searches conducted on Friday revealed any explosive devices, police and media reported.

However, the bomb threats caused disruption in the affected cities. Dozens of people were evacuated and court proceedings were interrupted as court buildings were searched and in most cases evacuated. Some adjacent streets were also closed temporarily.

Oliver Pohl, police spokesman in Kiel, said city officials received an emailed bomb threat late on Thursday that was signed with the words: "Nazi offensive."

He said there was no information thus far about the identity of the perpetrator, and authorities in affected cities were comparing notes.

The court in Kiel had received a separate bomb threat in December, although that one was delivered via a public "comment" website, not an email, a spokesman said.

A spokesman for the force covering Wiesbaden said it was too early to say if there was a link between the various bomb threats.

"The investigation just began," said Johannes Neumann.

A spokeswoman for Germany's Federal Criminal Police said it knew of the bomb threats solely through media reports and was not currently involved in any investigation.

The threats came after an attack in Leipzig on New Year's Eve, when tyres were set aflame outside the federal court building and a wooden emergency door was set on fire. Local officials estimated over €100,000 worth of damage was caused.

Bild newspaper reported last week that a radical leftist group had claimed responsibility.