The city in Germany's industrial Ruhr region is to be brought to a standstill by a mass evacuation affecting retirement homes, hospitals and the main train station.

In an online statement, Dortmund city hall said they had identified four locations where they suspect unexploded bombs dropped by Allied forces during World War II are buried.

The suspicions were based on "anomalies" detected during construction projects, the statement said.

"Only an excavation" could confirm the existence of bombs, it added.

Residents within a 500m radius of each location were told to leave their homes by 8:00 am (0700 GMT) on Sunday, with excavation work to begin later in the day.

Preparations were already in full swing on Saturday, with 58 patients evacuated by ambulance from two hospitals.

Residents at two retirement homes were also moved to safety a day ahead of the work.

The city hall tweeted images of containers being set in place to form a protective wall around the affected area.

The total area to be evacuated covers a large part of Dortmund's city centre, including the main train station and nearby National Football Museum.

The train station will be closed Sunday morning, with trains rerouted to avoid the city.

The unearthing of World War II era bombs is a common occurrence in Germany, but local media claimed this case could be the largest such evacuation ever seen in the Ruhr region.

In 2017, around 65,000 people were temporarily evacuated -- the largest such operation since 1945 -- when a huge British bomb weighing 1.4 tons was discovered in Frankfurt.

Last September, 15,000 people were evacuated from their homes while a 250-kilogramme (550-pound) device was defused in Hanover.