Turnout was a record low 46 per cent of the 44 million eligible voters, far less than in the last regional vote in 2004.
Results showed the Socialists set to crush Sarkozy's governing UMP party in Sunday's second-round polls, which is the last major ballot before the 2012 presidential vote.
In a vote seen as a key test of Sarkozy's popularity, the Socialists picked up 29.5 per cent in the first round in the 22 regions, ahead of the UMP with 26.3 per cent.
The outcome was seen as punishment by voters reeling from the global economic crisis that has sent French unemployment soaring to its highest level in a decade.
The anti-immigrant National Front, led by firebrand Jean-Marie Le Pen, made a stronger-than-expected showing when it picked up nearly 12 per cent of the vote.
The Elysee presidential palace made no public comment on the results, but Prime Minister Francois Fillon said he was hoping that a better turnout in the second round would help the UMP.
The Socialists moved to forge alliances with smaller parties ahead of round two. They held talks with the green party Europe Ecologie, which picked up 12.5 per cent of the vote.
The vote could boost Aubry's credibility as leader of the deeply divided party, which is set to hold primaries to choose a presidential candidate next year.
The final results will be decided in a second round of voting on 21 March. Only party lists with 10 per cent or more of Sunday’s vote are eligible for the runoffs, while those with between 5 and 10 per cent have until 6pm on Tuesday to merge their candidates with one of the qualified lists.