France's far-right National Front (FN) saw record gains in the first round of regional polls, held under a state of emergency just three weeks after 130 people were killed in attacks in Paris.
Despite the strong result, it faces an uphill battle to clinch a run-off vote next week after Socialists withdrew candidates to block it from power.
The FN came first with at least 30 percent of the vote nationwide and topped the list in at least six of 13 regions, according to estimates from the interior ministry.
FN leader Marine Le Pen and her 25-year-old niece Marion Marechal-Le Pen broke the symbolic 40 percent mark in their respective regions, shattering previous records for the party as they tapped into voter anger over a stagnant economy and security fears linked to Europe's refugee crisis.
Marine Le Pen, a lawyer by training, welcomed the "magnificent result", saying it proved the FN was "without contest the first party of France".
A grouping of right-wing parties took 27 percent, the official estimates showed, while the ruling Socialist Party and its allies took 23.5 percent.
The polls were held under tight security following France's worst-ever attacks, which have thrust the FN's anti-immigration and often Islamophobic message to the fore.
About half of the 45 million registered voters took part in the polls.
Any party that secures 10 percent backing in the first round has the right to present candidates in the second round, due to be held next Sunday.
Estimates showed 47-year-old Le Pen taking a whopping 40.5 percent of the vote in the economically depressed northern region of Nord-Pas-de-Calais-Picardie, once a bastion of the left.
Marechal-Le Pen did equally well in the vast southeastern Provence-Alpes-Cote d'Azur, known for its glamorous beaches and stunning countryside, on 40.5 percent.
The far-right success triggered an immediate debate among the mainstream parties as to whether, in regions where they trailed third, they should urge voters to back the candidate opposing the FN.
Socialist leader Jean-Christophe Cambadelis said his party would withdraw from the second round in the regions Le Pen and her niece were leading and called on its voters to back conservatives in order "to block" the FN.
But former conservative president Nicolas Sarkozy - head of the Republicans, which leads the right-wing grouping - repeated his refusal to do the same in key polls where the Socialists are ahead.
"We must hear and understand the profound exasperation of the French people," he said.
Le Pen said she was "not worried" by Socialist plans to withdraw but acknowledged that "things will obviously be a bit less straightforward".
FN party secretary Nicolas Bay, however, was more strident, saying that by apparently joining forces the two parties were "revealing themselves for the caricatures that they are".
'We told you so'
President Francois Hollande has seen his personal ratings surge as a result of his hardline approach since the November 13 attacks in Paris.
But his Socialist Party has languished behind the FN and the centre-right Republicans.
The FN's anti-EU and anti-immigrant narrative has been a lightning rod for many French who have lost faith in mainstream parties after years of double-digit unemployment and a sense of deepening inequality.
Victories next week would not only hand control of a regional government to the FN for the first time, but would also give Le Pen a springboard for her presidential bid in 2017.
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She has strived to "de-demonise" the party since taking over the reins in 2011, distancing herself from the more overt racism of her father - cofounder Jean-Marie Le Pen - going as far as to boot him out of the party this year.
The FN's repeated linking of immigration with attacks has also helped it climb in the polls since the gun and suicide bombing assaults in Paris.
When it emerged that at least two of the attackers had entered Europe posing as refugees, the FN aggressively pushed a message of "we told you so".