An unexploded American aerial bomb from World War II was found in central Berlin during construction work on Friday, forcing the evacuation of surrounding city blocks, police said.

The 100-kilogramme explosive was unearthed on a building site adjacent to the Alexa shopping centre and near a train line at the central transport hub of Alexanderplatz in the city's east.

People were being evacuated from within a 300-metre radius of the corroded bomb, which had an intact detonator, police said.

A police bomb disposal squad was set to attempt to defuse the bomb later in the day, police said on Twitter, warning that nearby areas would remain blocked off.

Nearly 75 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 in 2017 so that an unexploded 1.8-tonne British bomb dubbed the "blockbuster" could be defused.

Berlin was subject to a campaign of intense bombardment during the war, particularly in the spring of 1945, with a third of the city's homes destroyed and tens of thousands of people killed.

Thousands of items of unexploded ordnance have since been discovered, but some 3,000 are still believed to remain buried in the city's subsoil, according to experts.

In April 2018, Berlin police defused a 500-kilogramme British bomb, forcing the evacuation of 10,000 people.