PARIS (Reuters) - French Prime Minister Manuel Valls criticized Emmanuel Macron on Wednesday over the economy minister's thinly veiled presidential ambitions, accusing him of disloyalty in the face of serious threats facing the country.

In his most outspoken criticism leveled so far against his cabinet colleague, Valls laid bare the deepening tensions inside the government of unpopular President Francois Hollande, less than a year before parliamentary and presidential elections.

Speaking after Macron held a first rally for his recently founded political movement, promising a break with "the system", Valls implied that Macron had failed in his "duty of responsibility" to a country reeling from Islamist militant attacks.

"We have lived up to our responsibilities in the face of this threat," Valls said. "The duty of responsibility brings a duty of clarity, and to avoid exploiting ambiguity."

Macron, a 38-year-old former investment banker and presidential advisor, told his "En Marche" supporters on Tuesday he would carry the nascent movement to victory in next year's elections - without explicitly announcing a presidential bid.

"You're not very well placed to indulge in populist attacks on 'the system' when you are yourself an embodiment of the elite," Valls said.

Macron, who came to symbolize Hollande's 2014 pro-business U-turn, has been hinting for months at his loftier ambitions, to the mounting annoyance of rivals among the governing Socialists.

On Tuesday, to chants of "Macron for President!", he implicitly criticized his own government's timid economic reforms, vowing to shore up public support for deeper change across France's political landscape.

(Reporting by Laurence Frost and Michel Rose; Editing by Janet Lawrence)