Polish military divers on Tuesday defused a massive World War II bomb discovered at the bottom of a channel near the Baltic Sea, the navy said.

No one was harmed and nothing was damaged during the delicate operation to neutralise the five-tonne device -- nicknamed "Tallboy" and also known as an "earthquake bomb" -- even though there was an explosion.

The navy had earlier said it had ruled out the traditional option of a controlled explosion for fear of destroying a bridge some 500 metres away.

The bomb was discovered last year embedded at a depth of 12 metres (39 feet) during dredging close to the port city of Swinoujscie in the far northwest of Poland.

The device, which was dropped by the Royal Air Force in an attack on a Nazi warship in 1945, "can be considered neutralised," Grzegorz Lewandowski, spokesman for the Polish navy's 8th Coastal Defence Flotilla based in Swinoujscie, said in a statement.

More than six metres long, the bomb had 2.4 tonnes of explosives -- equivalent to around 3.6 tons of TNT.

An underwater image of the bomb
An underwater image of the bomb Polish Navy -- 8th Coastal Defence Flotilla / Handout

The navy had planned to use a technique known as deflagration to burn the explosive charge without causing a detonation, using a remotely controlled device to pierce through the shell to begin combustion.

But in the end "the deflagration process turned into a detonation," Lewandowski said, adding that "there had been no risk to the individuals directly involved."

A Swinoujscie city hall spokesman told AFP that he had not heard any reports of anyone injured during the operation, or of any damage done to the city's infrastructure.

Around 750 local residents had been urged beforehand to evacuate, while maritime traffic on the navigation channel and surrounding waterways was also suspended in the area.

During World War II, Swinoujscie -- at the time Swinemunde, a part of Germany -- was home to one of the German navy's most important Baltic bases and was subjected to massive bombardments.

On April 16, 1945, the RAF sent 18 Lancaster bombers from the 617th Squadron, known as the "Dambusters".

The bombers released 12 Tallboys on Germany's Lutzow cruise, including the one that failed to explode at the time.