Berlin, May 27 In the biggest evacuation since the Second World War, authorities in Germany's fourth-largest city of Cologne on Wednesday started evacuating around 20,000 people after a 1,000-kg bomb was found in the north-east area of the city.

According to a report in the German daily Bild, the evacuation process was taking place in the Riehl and Mülheim districts of town.

Schools in the area were closed. Bridges, streets, trains, boats and even planes were blocked, cancelled or rerouted, The Local news portal reported.

The bomb, an American explosive from the Second World War, was found during work on an underground heating system.

The operation to secure the bomb could take a long time, since it was found five metres underground.

Finds of this nature are not uncommon. Last year, two unexploded bombs had to be defused in the same Cologne neighbourhood.