Occupy activists interrupted Ackermann's speech at the chamber of commerce in Hamburg last month [EPA]

A letter bomb addressed to Deutsche Bank's chief Josef Ackermann has been intercepted at the bank's Frankfurt headquarters, German authorities say.

German police and bank officials said it was unclear who had sent the package, which was found in the bank's mail room on Wednesday.

"Initial investigations show that this was an operational letter bomb," police in the west German state of Hesse said in a statement on Thursday.

"What we had here was a powder, which indicates that we're not dealing with commercial or military explosives," Alexander Kiessling told the Reuters news agency. "It could be something homemade, for instance made out of fireworks."

Investigation begins

Kiessling said the powder in the envelope raised the suspicion that it could detonate but that state police now had
to examine the substance to determine what it was.

Police declined to say whether there was a detonator in the envelope that could have ignited the powder.

Security has been stepped up at Deutsche Bank offices around the world, banking sources said.

Ackermann, 63, has headed Germany's largest bank since 2002 and is seen as the face of capitalism in Germany.

"We have no indications that other countries are affected. There is no sign that there are other letter bombs," Udo Buehler, spokesman for the Criminal Investigations Office for the state of Hesse, told Reuters Television.

It is unclear who sent the explosive package and no one has so far claimed responsibility.

Activists of the Occupy movement, protesting against corporate greed, stormed the podium as he gave a speech at the chamber of commerce in Hamburg last month.

The Occupy movement started in the US but has spread across the world.