Le Pen said nevertheless that the FN, "against a bloc that represents the interests of the oligarchy, are the only force of resistance".
She said they will especially fight against what she called Mr Macron's pro-European, pro-migrant policies.
Polling agency projections suggested Macron's Republic on the Move!
Macron has said he wants to embody a "Jupiterian" vision of the presidency - whereby the president, very much like the Roman god of gods, speaks rarely except to issue orders.
Later, France's conservative leader declared his party the main opposition.
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Pollsters projected Republic on the Move! and its allies could take up to about 360 of the lower chamber's 577 seats.
The result redraws France's political landscape, humiliating the Socialist and conservative parties which alternated in power for decades until Mr Macron's election in May.
As ballots were still being counted, Macron's party held around 42% of the vote followed by the Republicans on 22%.
The party had enough seats to "defend its convictions", said the party's leader for the elections, Francois Baroin, calling on Macron to heed the record-low turnout, which he said sent "a message".
Macron's party, founded just a year ago, also won the first round of elections on June 11 with less than half of eligible voters going to the polls.
The Interior Ministry said the midday participating figures in Sunday's nationwide voting compare to 21.4 percent at midday during the second round of the last parliamentary elections in 2012.
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At the end of the afternoon, turnout stood at only 35 percent - below last week's record low. Overall, the first-round vote saw record low voter interest, with less than half of France's 47.5 million voters taking part overall. While REM is set to crush its rivals, the 39-year-old President could struggle to get his plans for far-reaching labour reforms past the fiery French streets.
Political heads of En Marche! are not under any illusions - Sylvain Maillard, a 45-year-old businessman elected by an absolute majority from the first round in Paris admits that they won because they "are the president's candidates", adding: "The French have carried the project of Emmanuel Macron, we must not deceive them, they voted for us because they expect change".
Still, prevailing sentiment among many on Sunday was that the newly elected president must receive a solid majority to implement the many reforms on which he campaigned.
Extreme-right National Front scored better than expected in recent polls, with eight seats in Parliament. Philippe is a convert to Macron's new party.
Le Pen, 48, who lost her presidential bid to centrist Emmanuel Macron, triumphed in her northern fiefdom of Henin-Beaumont, a depressed former mining town, the town's FN mayor Steeve Briois said.
The second and final round of legislative competition cleared the way for France's youngest top official to control the country's political power over five-year term.
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The huge majority gives Macron a robust mandate for reforms that include cutting tens of thousands of public sector jobs, making it easier for companies to hire and fire and capping redundancy packages.