A German bomb disposal team has successfully defused an unexploded World War II bomb that had forced the evacuation of 18,500 people in the city of Ludwigshafen.

The 500-kilogramme (1,100-pound) aerial bomb, thought to have been dropped by American forces, was discovered during construction work earlier in the week.

"Good news: the bomb has been defused! Citizens may return to their homes," the city of Ludwigshafen said on its official Twitter feed.

It also posted a picture of the freshly unearthed, corroded bomb, strapped to a pallet before being removed from the area.

Authorities in the western city had ordered all those living within a 1,000-metre radius of the bomb site to leave their homes early this morning as a precaution ahead of the defusing operation.

It took the bomb squad just over an hour to complete the delicate task.

More than 70 years after the end of World War II, Germany remains littered with unexploded ordnance, a legacy of the intense Allied bombing campaign against Nazi Germany.

In the biggest post-war evacuation to date, some 60,000 Frankfurt residents were evacuated last year so that an unexploded 1.8-tonne British bomb dubbed the "blockbuster" could be defused.

In April, thousands had to clear an area around Berlin's central railway station after another British bomb was discovered on a building site.

Unexploded munitions have also complicated the work of fire fighters this summer, with forest fires sparked by a spell of hot, dry weather setting off long-buried ordnance, causing small explosions on several occasions.