It was one of Germany's biggest bomb-related evacuations since the war ended with some 2,500 police officers, firefighters and paramedics on duty across the city to secure the operation.
Experts successfully defused the British 1.8 ton bomb and a 275-pound U.S. bomb that had been discovered last month after the Rhine river's water level fell significantly due to a prolonged lack of rain, said Heiko Breitbarth, a spokesman for Koblenz's firefighters.
Some 45,000 residents, living within a radius of about 2 kilometers (1.2 miles) from the bomb site had to leave their houses early Sunday before the evacuation order was lifted in the evening, the city said on its website.
Among those ordered to evacuate were seven nursing homes, two hospitals and a prison with some 200 inmates.
The British bomb could have cause massive damage in case it exploded.
"I did my job, that was all," lead defusing expert Horst Lenz told local daily Rhein Zeitung.
Finding unexploded bombs dropped by the Allies over Germany during World War II is common over 65 years after the war's end. The explosives are usually defused or detonated by experts without causing injuries.
Authorities in Koblenz had set up shelters for the evacuees and used buses to carry them to safety.
Train and road traffic came to a halt in the area, some 130 kilometers northwest of Frankfurt during the operation.
The residents of Koblenz, which was heavily bombed during World War II, are 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 are often found during construction work or by farmers plowing their fields.
Separately, another 200 people had to be evacuated from the southern German city of Nuremberg as experts there defused another bomb left from the war. The 70 kilogram (155 pounds) of unknown origin was defused in 15 minures, the city said in a statement.