"France is back", Prime Minister Edouard Philippe told French TV.
With 46 per cent of votes counted from yesterday's balloting, the interior ministry said En Marche had more than 26 per cent of votes.
François Baroin, head of the Republicans, said the low turnout testified to the "deep divisions in French society" and was "extremely worrying".
That a fresh-faced outsider could win a presidential election without a party machine was surprising. "What better spot to be useful to others than being a member of parliament".
Polls project it could win about a dozen seats, in part because of a voting system that favors the biggest parties.
The election took place amid heightened security after a series of devastating terror attacks in recent years.
Some experts say the low turnout reflects fatalism among Macron's opponents in the face of his seemingly unstoppable advance. They also blamed the long election cycle, with party primaries that started a year ago before the two rounds of presidential and then legislative voting, for turning voters off.
The FN's deputy leader Florian Philippot admitted to "disappointment" and called on voters to "mobilise massively" for the second round.
But his La France Insoumise took only 11 percent, and Kantar Public-onepoint forecast 13 to 23 seats for it and the Communist Party combined. His movement could obtain between 10 and 20 seats.
Cambadelis later confirmed he had been eliminated from the competition for his Paris seat, one that was previously a safe one for the Socialists.
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The University of Virginia student was accused in January 2015 of trying to steal a propaganda banner while visiting North Korea. And lawmakers from both sides of the aisle are urging consideration of a ban on American travel to the isolated country.
Macron has urged voters to back his reform proposals including an overhaul of the rigid rules governing the job market, blamed by many economists for holding back growth.
"We would have a National Assembly with no real power of control and without democratic debate to speak of", he said.
"It is neither healthy nor desirable for a president who gathered only 24 percent of the vote in the first round...and who was elected in the second round only by the rejection of the extreme right should benefit from a monopoly of national representation", Socialist party leader Jean-Christophe Cambadelis said.
Pollsters project Macron's alliance could win as many as three-quarters of the seats in the lower house after next week's second round of voting.
A host of opinion polls show Macron's party taking around 30 per cent of the vote today, putting it in pole position to secure a landslide in the second round next Sunday.
For the Socialists and their allies, the damage Sunday was even worse.
German Chancellor Angela Merkel congratulated Macron on a "great success" Sunday, calling it "a vote for reforms".
Mr Macron, 39, needs a majority to push through the changes that he promised in his campaign. Most had never held elected office and just five percent were outgoing parliamentarians.
His party fielded political newcomers in around 200 constituencies, some of whom felled heavyweights of the left and right in the first round.
The BBC's Hugh Schofield in Paris said everything suggested Mr Macron has won another crushing victory, with the only significant opposition coming from the centre-right Republicans.
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