Experts have defused two bombs, one reportedly the largest Second World War bomb ever uncovered in Germany, after the discovery sparked the evacuation of nearly half of the city of Koblenz.

Some 2,500 police officers, firefighters and paramedics were on duty across the city to secure the operation as 45,000 residents living within a radius of about two kilometres from the bomb site were told to leave. Five hundred emergency vehicles, including ambulances, were on the scene in the case of an explosion.

Authorities set up shelters in parts of Koblenz farther away from the bomb site, and shuttle buses were on hand in the morning to carry residents to safety. The evacuation order also affected people at two hospitals and seven nursing homes, as well as 200 inmates from a prison. 

Koblenz firefighter spokesman Heiko Breitbarth said the evacuation order still remains in place because a smaller smoke grenade found nearby will be brought to a controlled explosion.

The larger bomb — a 1.8 tonne ordnance dropped by the Royal Air Force between 1943 and 1945 — could have caused massive damage if it had exploded. It was found last week alongside a 125-kilogram U.S. bomb and a smoke grenade after the Rhine's water level fell significantly due to a prolonged lack of rain. All the devices were to be defused Sunday.

Finding unexploded bombs dropped by the Allies over Germany is common even more than 65 years after the war's end. The explosives are usually defused or brought to a controlled explosion without causing injuries.

28 bombs found in city since 1999

Officials had built a dam of hundreds of sand bags around the bomb site in the river bed to pump water out in preparation for the delicate task. Bomb experts started defusing the bombs early Sunday afternoon, Koblenz firefighter spokesman Heiko Breitbarth said.

Train and road traffic had come to a halt in the area, some 130 kilometres northwest of Frankfurt.

"Please close your houses and apartments, close your windows and, if possible, the shutters. Please think of bringing sufficient quantity of any medicine you might need," residents were advised by the city via leaflets and radio transmissions.

Several hundred city officials went from door to door Sunday morning, ringing the bells to check whether any residents had failed to evacuate the area.

The residents of Koblenz, which was heavily bombarded during the Second World War are somewhat used to bomb scares. City officials said 28 smaller war bombs had been found there since 1999, the German news agency DAPD reported. Such bombs in Germany are often found during construction work or by farmers plowing their fields.