Two World War II bombs, one of them a massive device known as a blockbuster, have been defused in the German city of Koblenz.

The bombs were discovered after a prolonged dry spell saw water levels fall in the River Rhine. The bigger of the two weighed 1.8 tonnes and was dropped by the Royal Air Force between 1943 and 1945.

Nearly half the city's population, 45,000 people, were evacuated, including patients at two hospitals, seven nursing homes and inmates at a prison.

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Three bomb disposal experts made the devices safe, while a smoke grenade canister containing dangerous chemicals was blown up in a controlled explosion.

It's the biggest bomb disposal operation in Germany since 1945, and disposal expert Marco Ofenstein said the operation was particularly risky.

"We have a British detonator which was surrounded by water for a long time and the explosives within the detonator react with water over time, which causes a high risk when the detonator is being removed," he said.

Hundreds of people had to be driven out of the evacuation zone earlier in the day, authorities said.

The evacuation was handled by 2,500 fire, police, medical and technical personnel as well as city officials. Seven schools were made available for shelter.

Loud-speaker announcements made from vehicles earlier warned residents to leave.

Leaflets had already been distributed advising "Please lock your houses/flats, close your windows and, if possible, leave the shutters on the windows down".

"Please think to take any necessary medication in sufficient quantity," the leaflet read.

Streets had been closed off and the main train station was earlier shut to trains.

A smoke grenade canister containing dangerous chemicals is blown up in a controlled explosion. ( AFP: Fredrik Von Erichsen )

Residents had spotted the British bomb from their balcony and had informed authorities, German newspaper Welt am Sonntag reported.

"The Rhine is full of bombs," one local said. "At least I assume it is. [With] the falling water levels, who knows how many bombs are still in there?"

More than six decades after the war, authorities believe there are still about 3,000 bombs buried beneath the capital Berlin alone, and unexploded devices are regularly discovered in construction work.

In response to Nazi air raids on civilian targets in Poland and later London, the Allies dropped about 1.9 million tonnes of bombs on Germany in an effort to cripple German industry. The allied raids killed about 500,000 people.

Last year three bomb disposal experts were killed while defusing a bomb in Lower Saxony.

A few months earlier, three other people died in the same area when a 500 kilogram World War II bomb was unearthed and exploded during the construction of a sports stadium.

ABC/AFP