Sixty police officers and commandos in Hamburg were called out after a jogger wearing a weighted vest was mistaken for a terrorist.
The German city was put on high alert when a passer-by mistook the runner for a suicide bomber and reported it to police.
The runner is said to have "danced around" at a traffic light before entering a nearby building, Deutsche Welle reported.
But police soon found out the man had only been on his daily workout, before returning to his office.
He was wearing the weighted vest to increase the intensity of the exercise, and jogged at the traffic lights to keep up his heart rate.
The area was soon cleared after police realised the mistake. Authorities are worried the incident could inspire others to do the same and create a nuisance, Deutsche Welle says.