French president Emmanuel Macron's parliamentary majority comes just weeks after his own presidential victory.
Polling agency projections suggested that Mr Macron's En Marche party could take 355 to 365 seats in the 577-seat National Assembly, the powerful lower house.
The turnout hit a new record low of just 43%, offering Le Pen the opportunity to question the legitimacy of the new parliament and the strength of the support Macron has received. In the presidential election, Le Pen won more than 50 percent of votes in her head-to-head with Macron in 45 voting districts and drew a total of 10.7 million votes, a historic high for the far-right party.
But he may be getting more than he bargained for with the entrance into parliament of loud voices from the ultra-left and of far-right National Front leader Marine Le Pen, both promising to fight his plans to overhaul French labor laws, one of the touchiest subjects in France.
Mr Macron's party, which did not exist 14 months ago and offered novice candidates from civilian life, has drawn from left and right to fill its ranks, effectively blurring the traditional left-right political divide.
Candidates from the conservative party, The Republicans, are expected to form the largest opposition group, with 70-110 seats, according to pollsters.
While French voters have handed past presidents large majorities in parliament, what's different this time is that Macron's party is splitting - and therefore weakening - the opposition. Overall, the first-round vote saw record low voter interest, with less than half of France's 47.5 million voters taking part overall.
Pollsters predict the party will see its strength in parliament fall from almost 300 seats to around 20 after their five years in power under president Francois Hollande.
German Chancellor Angela Merkel said she is looking forward to continuing her cooperation with Macron, while Foreign Minister Sigmar Gabriel said in a Twitter posting that Sunday's vote paves the way for reforms in France.
The LREM win is big enough to give the new president a good chance of weathering the inexperience and diversity of his new political army, and to push forward with bold and controversial labour reforms, says the BBC's Lucy Williamson in Paris.
As of late Sunday evening there were 156 women lawmakers elected to France's National Assembly, already more than ever before, and with 148 seats as yet undecided.
An exit poll released at 8pm local time (7pm BST) on Sunday has predicted Mr Macron's party will have a landslide victory with 352 seats along with its ally Mouvement Democrate.
Prime Minister Edouard Philippe has said Macron "has embodied trust, willingness and audacity". They urged voters not to allow too much power to be concentrated in the hands of one party and warned Macron's MPs would be mere yes-men who would rubber-stamp legislation.
Jean-Luc Melenchon, the movement's firebrand leader, won a seat in the southern city of Marseille on a pledge to lead resistance to Macron's radical labour market reforms. Low turnout figures have led to the Socialists suffering a humiliating defeat, with the Front National vote in meltdown due to infighting following Le Pen's presidential defeat in May.