Berlin - The discovery of an undetonated World War II bomb has sparked a mass evacuation in Steglitz in the southwestern reaches of the city. The 250kg explosive device was discovered Monday by construction workers on Oehlertring.

"The measures are just starting here now," police spokesperson Heidi Vogt told the Berliner Zeitung at 4 pm. Initially, an area 300 metres around the dud will be set up with it extended to 500 metres during defusing.

Some 150 officers are helping with the operations and the Steglitz-Zehlendorf chapter of the German Red Cross said it would set up emergency shelter for residents who need shelter during the activity.

"I was alerted by a police car making announcements," said a 17-year-old home alone in the neighborhood Monday. She noticed her neighbors hurrying away and decided to call her parents. She then packed a few things: clothes, vaccination card, iPad.

The situation is now calmer but police are still swarming the area.

"That's when it popped into my head: the cops are going to come in and my room is a mess." But she was lucky: her house is outside the exclusion zone.

Swimming pool, gas station, grocery store

Employees at the outdoor Insulaner pool asked swimmers and sunbathers to leave by loudspeaker.

"Emergency personnel are already talking to residents to prepare them for the defusing," Vogt explained. A supermarket and a gas station were also evacuated alongside the pool, he said. But getting everyone out of the nearby apartments in time is likely more difficult.

Specialists at the scene have decided to disarm the bomb Monday evening, though no exact time is yet known.

"We'are optimistic we can do it in the evening," said Vogt.

The S-Bahn between the stations Priesterweg and Südende will also be halted during the defusing itself.