An alliance of French left-wing parties was on course Sunday to become the biggest parliamentary bloc by beating the far right and President Emmanuel Macron's coalition, according to surprise projected results.
No one group won an absolute majority in the poll, plunging France into political limbo with no clear path to forming a new government, two days before a major NATO summit and three weeks before the Paris Olympics.
The New Popular Front (NFP) -- formed last month after Macron called snap elections -- brought together the previously deeply-divided Socialists, Greens, Communists and the hard-left together in one camp.
Nevertheless, veteran presidential candidate Marine Le Pen's far-right National Rally (RN) led the race after the June 30 first round of voting, with opinion polls predicting that she would lead the biggest party in parliament after Sunday's run-off.
But projections based on vote samples by four major polling agencies on Sunday showed no group on course for an absolute majority and the left-wing NFP ahead of both Macron's centrist Ensemble and Le Pen's eurosceptic, anti-immigration RN.
Macron, who has yet to speak in public about the projections, is calling for "prudence and analysis of the results", said an aide, asking not to be named.
Firebrand leftist Jean-Luc Melenchon, leader of the hard-left France Unbowed (LFI) and the controversial figurehead of the NFP coalition, demanded that Prime Minister Gabriel Attal resign and that the left be allowed to form a government.
"The New Popular Front is ready to govern," he declared, in an address to supporters.
"Its constituent parts, the united left, have shown themselves equal to the historic occasion and in their own way have foiled the trap set for the country. In its own way, once again, it has saved the Republic."
The left-wing group was predicted to take between 172 and 215 seats, the president's centrist allies are on 150 to 180 and the National Rally in a surprise third place on 115 to 155 seats.
This marks a new high water mark for the far right, but falls well short of the victory they had hoped for, which would have seen Le Pen's 28-year-old lieutenant Jordan Bardella become prime minister. Instead, he expressed fury.
Bardella dubbed the local electoral pacts that saw the left and centrists avoid splitting the anti-RN vote as "alliance of dishonour" that had thrown "France into the arms of Jean-Luc Melenchon's extreme left".
"I say this tonight with gravity. Depriving millions of French people of the possibility of seeing their ideas brought to power will never be a viable destiny for France," he said, vowing to carry on the fight.
Last week saw more than 200 tactical-voting pacts between centre and left wing candidates in seats to attempt to prevent the RN winning an absolute majority.
This has been hailed as a return of the anti-far right "Republican Front" first summoned when Le Pen's father Jean-Marie faced Jacques Chirac in the run-off of 2002 presidential elections.
Macron will attend the upcoming landmark NATO summit in Washington a diminished figure and France has been left without a stable ruling majority less than three weeks before Paris hosts the Olympics.
The election campaign, the shortest in French history, was marked by a febrile national mood, threats and violence -- including racist abuse -- against dozens of candidates and canvassers.
Some 30,000 police were deployed to keep order, and many voters expressed fears that rioting could erupt in some cities after the results were announced.
Turnout was nevertheless high, with left-wing and centrist candidates urging supporters to defend democratic values and the rule of law -- while the far right scented a chance to upend the established order.
An outright RN victory would have weakened France's international standing and threaten Western unity in the face of Russia's invasion of Ukraine.
EU officials, already learning to deal with far-right parties in power in Italy and the Netherlands and frustrated by Hungary's Prime Minister Viktor Orban, are watching France closely.
The question for France now is if this alliance of last resort can now support a stable government, dogged by a huge RN bloc in parliament led by Le Pen herself as she prepares a 2027 presidential bid.