ATHENS — In an appearance filled with symbolism on his first state visit to Greece , President Emmanuel Macron of France on Thursday outlined his vision for a stronger, more united Europe from the heart of the country whose severe financial problems threatened the viability of the euro.

“I want to talk to you about a new Europe,” Mr. Macron he said, proposing a “road map” for the “rebuilding of the European Union.”

In an impassioned speech beneath the Acropolis, on Pnyx Hill, a meeting place for popular assemblies in antiquity, Mr. Macron elaborated on his vision for a stronger union. “Today in Europe, sovereignty and democracy are under threat,” he said, calling for national conventions to debate “what kind of Europe we want.”

“We must not be scared,” he said, “of this extreme ambition.”

Europe’s challenges, Mr. Macron added, include “how to become a power that can face the U.S. and China.”