Robbers used a large drill to break into a German bank's vault room during the extended Christmas break and steal cash, gold and jewellery worth 30 million euros ($35 million), police and the bank said Tuesday.

The spectacular heist in the western city of Gelsenkirchen, North Rhine-Westphalia state, saw thieves smash open more than 3,000 safe deposit boxes and make off with the loot.

While the criminals remained at large, hundreds of distressed bank customers massed outside the branch on Tuesday demanding information.

According to police, the robbers drilled their way into the underground vault room of the Sparkasse savings bank from a parking garage. 

The bank said the branch was "broken into over the Christmas holidays" and that "more than 95 percent of the 3,250 customer safe deposit boxes were broken into by unknown perpetrators".

German businesses were closed for Christmas on Thursday and Friday last week, and investigators suspect the gang may have spent the holidays and weekend inside, breaking open the deposit boxes.

The robbery came to light after a fire alarm was triggered in the early hours of Monday and emergency services discovered the hole. 

Witnesses reported seeing several men carrying large bags in the stairwell of the parking garage during the night from Saturday to Sunday, police said. 

Footage from security cameras showed a black Audi RS 6 leaving the parking garage early Monday morning, with masked persons inside. 

'Professionally executed'

The car's licence plate had been stolen earlier in the city of Hanover, police said. 

A police spokesman told AFP that the break-in was "indeed very professionally executed", likening it to the heist movie "Ocean's Eleven". 

"A great deal of prior knowledge and/or a great deal of criminal energy must have been involved to plan and carry this out," he said. 

Police said the thousands of boxes had an average insurance value of more than 10,000 euros, and therefore estimated the damage at some 30 million euros.

Several victims had told police that their losses far exceeded the insured value of their safe deposit boxes.

The police spokesman said that "disgruntled customers" were outside the bank branch which did not open for security reasons, after threats had been made against the employees.

"We're still on site, keeping an eye on things," he said, adding that "the situation has calmed down considerably".

The bank said it had set up a hotline for customers and that all those affected would be informed in writing as soon as possible while it was working with the insurance company to determine how to process the claims.

"We are shocked," said bank press spokesman Frank Krallmann. "We are standing by our customers and hope that the perpetrators will be caught."