French President Francois Hollande's Socialists and their allies are headed for the absolute majority needed to push through economic reforms after the first round parliamentary vote, exit polls show.
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 on Sunday put the Socialists and their Green allies at about 40 per cent, ahead of the 35 per cent score of ex-president Nicolas Sarkozy's right-wing UMP party.
TNS Sofres, Ipsos and OpinonWay pollsters all agreed the two parties would win at least 287 seats in the 577-seat National Assembly and possibly as many as 347, with potential allies in the Left Front taking 13-20 seats.
Mr Hollande defeated Mr Sarkozy in last month's presidential election and wants voters to give him a strong mandate to enact reforms as France battles Europe's debt crisis, joblessness and a stagnant economy.
If next week's second round confirms Sunday's results, it will boost his status in Europe as champion of the movement away from German-led fixation on austerity towards growth, which he favours as the solution to the economic crisis.
Mr Hollande's Prime Minister, Jean-Marc Ayrault, called for a "large, solid and coherent majority" for the Socialist party and its allies.
"Change is going to be around for a while," he said, echoing the Socialists' presidential election slogan.
The CSA poll gave the National Front more than 13 per cent, against four per cent in the 2007 parliamentary election, but under France's first-past-the-post system, that would at best give it only three parliamentary seats and possibly none.
The Communist-backed Left Front, headed by firebrand Jean-Luc Melenchon, was set to win7 per cent of the votes. CSA put turnout nationwide at about 60 per cent.
But the night marked a personal defeat for Mr Melenchon, who took Ms Le Pen head-on in a bitter battle in a rundown former mining constituency near the northern city of Lille.
Ms Le Pen claimed her victory meant her party was now France's third political power.
"Given the abstention rate and a profoundly anti-democratic electoral system that has for 25 years deprived millions of voters of MPs, we confirm our position tonight as France's third political force," Ms Le Pen said.