The enormous bomb, dropped by the Royal Air Force, was only discovered thanks to the unusually low level of the Rhine, due to a severe lack of rain over the last few weeks.
The bomb disposal situation has been complicated by the discovery of two more, smaller bombs in the mud nearby.
The Koblenz city council said around 45,000 people will have to move, that is around 42 percent of the city's population, in what will be the biggest evacuation in its history.
An evacuation radius of 1.8 kilometres has been decided upon. This means 700 patients at two hospitals will have to be moved, as well as the residents of seven old people's homes and prisoners in a jail. The city's main train station will also have to be emptied as well as several hotels.
The evacuation zone will have to be empty from 9 am, while the bomb should be defused between 3 pm and 5 pm, after which people will be able to return to their homes.
The low level of the Rhine is exposing unexploded World War II bombs which have lain under water for decades.
Police closed a road and a stretch of railway near Rhein bei Vallendar near Koblenz on Saturday so that three smoke bombs found in the river could be blown up. An artificial traffic jam was created on a nearby autobahn to slow traffic while the explosion work was underway.
A day later and just a few kilometres downriver the bomb disposal crews had their work cut out for them in Neuwied where a 500-kilo aerial bomb was spotted in the river. This required the evacuation of around 1,000 people living in the nearby area of Neuwied.