Macron joins race to 'unblock France’
The political system was paralysed, he said. "I want to free the energy of the able while protecting the weakest".
Despite serving under President Francois Hollande, the centrist is running as an independent candidate.
His move has unsettled rivals from both the left and right hoping to represent the two mainstream parties.
The conservative Republican party will select its candidate in new US-style primaries from Nov.13.
Earlier, frontrunner Alain Juppe accused Mr Macron of betraying President Hollande, and said he had "stabbed him in the back".
A one-time protege of the president, Mr Macron quit the Socialist government in August amid discontent within the party over his political ambitions.
Mr Hollande has not yet confirmed whether he will stand for re-election in April, and will make a decision in the coming weeks. He has the lowest popularity ratings of any post-war French president.