François Hollande, who will stand as the Socialists' candidate in next year's presidential elections, was in Brussels for a series of meetings with fellow left-wing parties.
He criticized a German proposal that would place the supervision of national budgets under the auspices of the European Court of Justice.
"I will never accept that for the sake of controlling national budgets and coordinating budget policy the European Court of Justice should be the judge of the spending and receipts of a sovereign state," he said.
Hollande also took the opportunity to criticize the dynamics in the relationship between Merkel and French president Nicolas Sarkozy.
"For several months, it's Mrs Merkel who decides and Nicolas Sarkozy who follows," he said.
Hollande's comments seemed restrained in comparison with heated remarks made by fellow Socialist Arnaud Montebourg.
Montebourg, who came third in the October election to choose the party's presidential candidate, was speaking on a TV discussion programme on Wednesday about the eurozone crisis.
Asked about the German reluctance to let the European Central Bank act as lender of last resort, he said that Merkel had decided to impose a "German order" on other countries. He said Merkel was in the process of "killing the euro" and that the moment had arrived to "confront Germany and defend our values."
"The question of German nationalism is in the process of reemerging through the Bismarck-style policies of Mrs Merkel," he said, adding that the Chancellor was "building confrontation to impose her domination."