The FN topped the polls by winning the backing of just over one in four voters.
With most ballots counted following Sunday's vote, the Interior Ministry announced that the anti-immigration, anti-EU party led by Marine Le Pen had secured 24.96 percent of votes cast, guaranteeing them 24 seats, around a third of France's 74 places in the 751-member European Parliament.
The FN beat the centre-right Union for a Popular Movement (UMP) into second place with 20.8 percent of the vote, securing it 20 seats
President Francois Hollande's Socialist Party was left languishing in third with a humiliating tally of just 13.98 percent.
President François Hollande called in key ministers for a crisis meeting this morning.
Earlier on French radio, Prime Minister Manuel Valls said the stinging reverse demonstrated the need to accelerate political and economic reforms.
"Lessons have to be learned," a Hollande aide acknowledged. "We have to find a way of convincing the French people that we can change Europe without leaving Europe."
The result is the highest score ever obtained in a nationwide election by the National Front (FN) and follows breakthrough gains made by the once-pariah party in municipal elections earlier in the year.
As FN leaders celebrated their triumph by demanding the dissolution of the National Assembly, senior Socialist minister Segolene Royal acknowledged that the far right's success represented "a shock on a global scale."
Marine Le Pen, 45, has been credited with significantly broadening the appeal of a party founded by her father Jean-Marie Le Pen and long tainted by association with his multiple convictions for inciting racism and denying the holocaust.
She said voters had demonstrated their desire to "reclaim the reins of their own destiny."
"Our people demand only one type of politics - a politics of the French, for the French and with the French," she said.
"They have said they no longer want to be ruled from outside, to have to submit to laws they did not vote for or to obey (EU) commissioners who are not subject to the legitimacy of universal suffrage."
Although the FN is on the rise, political analysts continue to consider the prospect of an FN president in 2017 as extremely unlikely but many see French politics being transformed into a three-party system in which Le Pen's party could wield considerable influence.
The far right party's resurgence over the last few years has been attributed to the appeal of its core messages on immigration and Europe at a time of record high unemployment and falling living standards for many working and middle class voters.
But the FN has also benefited hugely with widespread disillusionment with the mainstream parties.
Both the UMP, whose former leader Nicolas Sarkozy ran France from 2007-12, and the Socialists have been beset by a series of scandals over alleged corruption or cronyism, as well as being seen by many as having failed to address France's problems.
Manuel Valls, who was promoted to prime minister after the Socialists were routed at the municipal elections in March, said Sunday's vote was a sign the government needed to accelerate the pace of reforms aimed at creating a more business-friendly climate in France, trimming the size of the state and getting the economy going again.