A fire alarm summoned police officers and firefighters to the bank branch shortly before 4 am on Monday.
Thieves stole tens of millions of euros worth of property from safety deposit boxes inside a German bank vault that they drilled into on Monday during the holiday lull, police said.
Some 2,700 bank customers were affected by the theft in Gelsenkirchen, police and the Sparkasse bank said.
Thomas Nowaczyk, a police spokesperson, said investigators believe the theft was worth between €10-€90 million.
German news agency dpa reported that the theft could be one of Germany's largest heists.
The bank remained closed on Tuesday, when around 200 people showed up demanding to get inside, dpa reported.
A fire alarm summoned police officers and firefighters to the bank branch shortly before 4 am on Monday.
They found a hole in the wall and the vault ransacked. Police believe a large drill was used to break through the vault's basement wall.
Witnesses told investigators they saw several men carrying large bags in a nearby parking garage over the weekend.
Video footage from the garage shows masked people inside a stolen vehicle early on Monday, police said.
Gelsenkirchen is about 192 kilometres northwest of the city of Frankfurt, which is the financial centre of both Germany and Europe and is home to the European Central Bank.