The chancellor said Germans have a "duty" to stand up to right-wing extremists just as the resistance faced down Adolf Hitler. July 20 is the 75th anniversary of the failed assassination attempt on the Nazi dictator.
"We, too, have a duty today to stand up against all those tendencies that want to destroy democracy," the chancellor added.
July 20 marks the anniversary of the 1944 bomb plot to murder Hitler and remove the Nazi Party from power. The attempt failed and Hitler survived with minor injuries. The plot's orchestrator, Claus Schenk Graf von Stauffenberg, and some 200 co-conspirators were executed.
Assassins were role models
Merkel said Germany owes them a debt of gratitude, noting that the country's post-war constitution may not have come into existence without such actions.
Lübcke had become a target of threats because of his support for Merkel's refugee policies.
The chancellor said his killing showed how important it was to support those who assume political responsibility — whether at the local level, as politicians or in non-governmental organizations and in society as a whole. "It requires a clear response from everyone."
All Germans were invited "to do their part in our society to ensure that democracy is strong, that civil society is strong and that right-wing extremism has no chance."