The conservative alliance, including Sarkozy's opposition UMP, took 32.5 per cent of the vote according to the latest polls, ahead of the governing Socialist Party led by much maligned President Francois Hollande.
The anti-EU and anti-immigration National Front (FN) led by Marine Le Pen, which had dominated the airwaves during the campaign, fell short of polls that had put it in the lead in the run up to the elections.
The far-right party led the first-round vote in 43 out of 98 "departments" -- which have power over local issues such as school and welfare budgets -- that voted in Sunday's poll, according to interior ministry figures.
The FN is expected to go through to the second round in more than half of the 1,100 "cantons" -- an administrative division below "departments" -- that will vote again in a week, an AFP analysis of the data showed.
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That put it ahead of Hollande's ruling Socialist Party, whose failure to address double-digit unemployment has seen him haemorrhage support since he took charge in 2012.
Provisional interior ministry figures gave the Socialists and their left-wing allies around 22 per cent of the vote.
The mainstream parties will be able to call on smaller allies when voters return for run-off elections next Sunday, while the FN will struggle to find partners.
"There will be no local or national deal with the leaders of the FN," declared Sarkozy immediately after the initial figures were released.