In the biggest event 80,000 people demonstrated in Berlin, according to police, while other protests in the cities of Stuttgart, Frankfurt, Munich and Dortmund each drew at least 20,000 demonstrators.
Among the contested reforms are changes to the public health insurance system and a plan to raise the retirement age from 65 to 67. Demonstrators also criticised a shortage of professional apprenticeships.
Saturday's demonstrations, organised by several of the country's leading unions, came amid a wider debate over social welfare in Germany. Reforms to health insurance have even divided parties within Merkel's ruling coalition.
"I can only advise the (federal government's) grand coalition not to ignore this cry for help," the president of the German Unions Confederation, Michael Sommer, said in Stuttgart.

Copyright AFP (Agence France-Presse), 2006