French President Francois Hollande's Socialists and allies were headed for an absolute majority to push through tax-and-spend reforms after a first round of parliamentary elections Sunday, exit polls said.
The election also saw a surge in support for Marine Le Pen's far-right National Front, which wants to ditch the euro and battles against what she calls the "Islamisation" of France.
Estimates released by pollster CSA after voting ended put the Socialists and their Green allies at around 40 percent, well ahead of the 35-percent score of ex-president Nicolas Sarkozy's right-wing UMP party.
Hollande defeated Sarkozy in last month's presidential election and wants voters to give him a strong mandate to enact reforms as France struggles with Europe's debt crisis, rising joblessness and a stagnant economy.
If Sunday's results are mirrored in next week's second round, that will boost Hollande's status in Europe as champion of the movement away from the German-led fixation on austerity. He favours promoting growth as the solution to the continent's lingering economic crisis.
The CSA poll put the National Front, whose leader got 18 percent of the votes in the first round of the presidential election, at more than 13 percent, far above the 4.0 percent it got in the last parliamentary election in 2007.
The Communist-backed Left Front headed by firebrand anti-capitalist Jean-Luc Melenchon was set to win seven percent of votes, CSA said, with turnout nationwide at around 60 percent.
As opinion polls had indicated that 40 percent of election-weary voters might not bother to cast their ballot, the president appealed to electors before Sunday's ballot to exercise their democratic right.
"I will only be able to bring about change, the change that the French have asked me to bring about, if I have a majority in the National Assembly," he said Thursday.
France's Socialists already control the upper house of parliament, the Senate.
But after taking 51.6 percent of the vote in the May 6 presidential run-off, Hollande moved quickly to give the Socialists the edge in the parliamentary elections to capture the lower house, the National Assembly.
Prime Minister Jean-Marc Ayrault's interim government has taken a series of popular steps in the wake of Hollande's presidential victory.
He has cut ministers' salaries by 30 percent, vowed to reduce executive pay at state-owned firms and lowered the retirement age from 62 to 60 for some workers.
But the UMP has hit back with warnings that the Socialists are preparing huge tax hikes to pay for what the right says is a fiscally irresponsible spending programme.
UMP leader Jean-Francois Cope said the Socialists were readying "the biggest-ever tightening of the screws on the middle class", while ex-premier Francois Fillon said the party would "massively boost taxes" with a majority.
Fillon sought to spin Sunday's results as showing that voters had not massively approved Hollande's time in office since he won the May 6 presidential election.
"First of all, there was tonight no 'pink wave'," he said in reference to the Socialists' sweeping 1981 parliamentary victory after their last president, Francois Mitterrand, was elected.
"For the president's party, there is no appetite for his project," Fillon said.
The economic backdrop is bleak for whoever wins the parliamentary vote, with unemployment at 10 percent, stalled growth, and a resurgent eurozone crisis.
More than 6,500 candidates were competing in Sunday's vote, which takes place over two rounds under a constituency-based simple majority system.
If no candidate wins more than 50 percent in the first round, any contender who scores more than 12.5 percent of the vote stays in the race for the second round.
Some 64,000 polling stations opened at 8:00 am (0600 GMT) on Sunday with the last stations closing in the large cities at 8:00 pm.