'This forms part of a generally increased level of threat ahead of the general election,' security agents said.

Although Germany's NATO-led mission in Afghanistan is unpopular domestically, only the hard-left Die Linke are calling for immediate troop withdrawal.

Harrach, a German of Moroccan origin, is already known to German security agents. He has appeared in several Al-Qaeda propaganda videos that have been published online this year.

In the latest video, Harrach reportedly warned Muslims in Germany to stay away from public spaces in the two weeks following the election.

Omer O, a Turkish Al Qaeda suspect on trial in the German city of Koblenz, is thought to have recruited Harrach to join the radical Islamist group.

German courts restrict the reporting of defendants' surnames.