COMPOSITE: In this digital composite image a comparison has been made showing crowds saluting to Adolf Hitler passing by in a motorcade on October 2, 1938 (Photo by Keystone/Hulton Archive/Getty Images) and a view from approximately the same perspective along on April 30, 2015 in Berlin, Germany. (Photo by Sean Gallup/Getty Images) *** ARCHIVE BLACK AND WHITE *** BERLIN, GERMANY - OCTOBER 2, 1938: Crowds salute German Nazi leader Adolf Hitler (1889 - 1945) on his arrival in Berlin from Munich, 2nd October 1938. (Photo by Keystone/Hulton Archive/Getty Images) *** MODERN DAY COLOR *** BERLIN, GERMANY - APRIL 30, 2015: Cars and cyclists pass through the intersection of Wilhelmstrasse and Leipziger Strasse on April 30, 2015 in Berlin, Germany. Europe, Russia and the former Allied powers will commemorate the 70th anniversary of the end of World War II and the victory over Nazi Germany on May 8-9. (Photo by Sean Gallup/Getty Images) 
By Rachel More
An evacuation zone with a radius of 1,000 metres (1,100 yards) was cleared from 8 a.m. (0600 GMT), 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 on Wednesday.
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 RRTL.DE, 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.
(Reporting by Rachel More, Editing by Ludwig Burger and Ed Osmond)