She returned from summer holidays "with a great determination and a burning sense of duty not for me but for you, not alone, but with you", she said.
Speaking to a crowd of about 500 in the northeastern town of Brachay, an FN stronghold, the populist leader said: "Our political family is the only one capable of embodying" a force that could counter the new centrist movement of President Emmanuel Macron.
Le Pen garnered 34 percent of the vote against 66 percent for Macron in the May presidential runoff.
In June, the anti-EU, anti-immigration FN went on to win eight seats in the 577-seat parliament. While the result was a historic high for the FN, many within the party were deeply disappointed, faulting Le Pen for running a poor campaign.
The party split over its key policy of wanting to scrap the euro -- seen as too risky by many voters, particularly from the older generation.
The radical left Unbowed France party of Jean-Luc Melenchon, with 17 seats in parliament, touts itself as the country's leading opposition force.
But on Saturday, Le Pen, 49, said: "We are the exact antithesis of Macronism."
She lambasted the president for what she called a "policy of perpetual precariousness", in a reference to his reforms to the labour code that will make it easier for employers to hire and fire staff.
"Macronism is the triumph of the dominant class whose only moral veneer is human rights and whose only values and purpose is money," she said.
The stunning success of Macron's centrist Republic on the Move party redrew France's political landscape, sidelining traditional left and right parties that had alternated power for decades.
As a result, no party has the clear profile of a political opposition, in the opinion of 39 percent of respondents to a poll published Saturday.
Unbowed France was cited by 32 percent, while 14 percent pointed to the FN. The right-wing Republican scored nine percent, according to the poll by BFMTV.