Friedrich Merz, the conservative frontrunner in the German election, is focusing his campaign on proposals to revive the struggling economy after a bruising clash last week over tougher immigration rules.

Former German Chancellor Angela Merkel has issued rare public criticism of her successor as the country's center-right leader, the front-runner in Germany's election next month, for putting to parliament proposals for tough new migration rules that only passed with the help of a far-right party.

Friedrich Merz, the favourite to become Germany's next chancellor, said on Monday that he is ready to lead the country as his conservative Christian Democratic Union (CDU) assembled in Berlin for a party conference weeks ahead of national elections.