An evacuation zone with a radius of 1,000m was cleared from 8am, impacting around 20,500 residents along with many workers and hotel guests in the city’s old town and Deutz district.

Three American bombs, each with impact fuses, were found during construction work on Monday in Deutz, a bustling area on the bank of the River Rhine.

Bomb disposal experts plan to disarm the ordnance later today.

Unexploded bombs are often found in Germany, where many major cities sustained heavy damage during the war.

The evacuation area includes one hospital, two retirement homes, nine schools, and many hotels and museums.

“Everyone involved hopes that the defusing can be completed in the course of Wednesday. This is only possible if all those affected leave their homes or workplaces early and stay outside the evacuation area from the outset on that day,” the city authority said in a statement.

The measures caused major transport disruptions in the city of over a million people, with Germany’s national rail operator warning that many trains would be diverted or cancelled.

A stretch of the Rhine will be blocked off before the bomb disposal operation begins.

The Rhine, which runs from the Swiss Alps to the North Sea via Cologne, is one of Europe’s key waterways for the transportation of commodities such as grain and coal.

Private television station RTL, whose main office is located in the evacuation zone, interrupted its morning news programme.

“We have to leave,” the news anchor said, grabbing his bag as the lights were turned off.