In an interview with the Frankfurter Allgemeine Sonntagszeitung (FAS), Steinbrueck said austerity measures were pushing some countries to do too much too soon. He said there would be massive protests in Germany if such a heavy dose of austerity were to be imposed so quickly.

"The savings measures are too severe, they're leading to depression," said Steinbrueck, 65, a Social Democrat (SPD) who was finance minister from 2005 to 2009 in Merkel's right-centre grand coalition government.

"Some societies are being forced to their knees. Budget consolidation is in some ways like medicine. The right amount can save lives while too much can be lethal."

Steinbrueck noted that some countries were being forced to make spending cuts that amounted to five percent of their gross domestic product (GDP).

"In Germany that would amount to 150 billion euros (of spending to be cut)," Steinbrueck said. "You can imagine what the protests would be like on German streets with that."

Steinbrueck said he and the centre-left SPD were clearly in favor of efforts to stabilize the euro zone - due partly to German national interests and also due to Germany's responsibility to the European Union.

"My advice to the SPD is that we shouldn't treat the European issue in the election campaign as a minor topic or without being courageous," Steinbrueck said, referring to fears in the SPD that talking about the euro zone rescue efforts would benefit Merkel more than the SPD.

"Europe has to be stabilized out of our national interests and out of our responsibility to Europe."

(Reporting By Erik Kirschbaum. Editing by Jane Merriman)

• A Permanent Economic Emergency