German explosives experts defused a massive World War II bomb in the financial capital of Frankfurt on Sunday after tens of thousands of people evacuated their homes in case it detonated.

The 1.8-tonne bomb was discovered on a building site last week, and more than 60,000 people were ordered to leave their residences in what is Germany's biggest evacuation since the war.

The work by bomb technicians started later than planned, as some people refused to leave the evacuation area despite authorities warning that an uncontrolled explosion would be big enough to flatten a city block.

Helicopters with heat-sensing cameras circled to spot stragglers, and police chiefs said they would use force if necessary to clear the area.

More than 1,000 emergency service workers helped to clear the area and a steady flow of people filed into a temporary shelter at Frankfurt's trade fair site.

The bomb was dropped by Britain's Royal Air Force during the 1939-45 war.

The explosive device - nicknamed "blockbuster" by German media - was found last week in the city's leafy Westend neighbourhood.

RELATED: 50,000 evacuate German city over unexploded WWII bombs

A spokesman for Frankfurt's firefighter service said earlier that people slowly moving out had delayed the operation, adding that was "incredibly annoying and time-consuming".

Police set up cordons around the evacuation area, which covered a radius of 1.5km.

Bomb disposal experts will use a special system to try and unscrew the fuses attached to the HC 4,000 bomb from a safe distance. If that fails, a water jet will be used to cut the fuses.

British and American warplanes pummelled Germany with 1.5 million tonnes of bombs that killed 600,000 people. Officials estimate 15 percent of the bombs failed to explode, some burrowing six metres deep.

More than 2,000 tonnes of live bombs and munitions are found each year in Germany.

Bombs everywhere

On Saturday, 21,000 people had to be evacuated from the western city of Koblenz as bomb disposal experts defused an unexploded American World War II shell.

In May, 50,000 residents were forced to leave their homes in the northern city of Hanover for an operation to defuse several WWII-era bombs.

David Hoffmann, 29, who works at a bank, was putting his luggage in his car on Sunday.

"I have the essentials with me - the most important documents," he said, complaining he had received no leaflets about the evacuation.

Although police have said there is no immediate danger, the bomb's massive size prevents them from taking any chances during the disarming process.