Germany's Social Democrats said they would go into opposition rather than enter a new coalition with Chancellor Angela Merkel's party after they suffered their worst post-war results in Sunday's election.

"We will take on the opposition mandate," said SPD deputy chief Manuela Schwesig after exit polls gave it 20 to 21 percent. 

Thomas Oppermann, who leads the SPD in parliament, confirmed that "the place of the SPD is in the opposition".