President Jacques Chirac's party has been humiliated in French regional polls, amid speculation that PM Jean-Pierre Raffarin could be sacked.
It was a night for celebration for the left
The socialists and their allies won 50% of the second-round votes, leaving Mr Chirac's centre-right governing UMP party trailing on 36.9%.
The socialists held eight councils and grabbed another 12 from the UMP.
Voters are thought to be angry at high unemployment, a stagnant economy and unpopular public sector reforms.
The centre-right has won only Alsace so far, with the results from Corsica still to come.
The election was seen as a mid-term test of opinion on the Chirac government.
Regional assembly wins:
Corsica and overseas results not included
Mr Raffarin said lessons had to be learnt by the government, but "reforms must be continued, very simply because they are necessary".
The first round last Sunday saw the UMP lose heavily to the socialists and their communist and green allies.
Socialist party leader Francois Hollande says voters have expressed their rejection of both Mr Raffarin's government and Mr Chirac.
Our correspondent says the result is expected to lead to a major cabinet reshuffle, with Mr Raffarin tipped as the first to lose his job.
One of the high-profile casualties already claimed by the poll is former President Valery Giscard d'Estaing, who lost the presidency of the Auvergne region to a socialist.
The far-right National Front confirmed its position as the country's third political force, with nearly 13%.
At least 60% of the voters in France said they remained determined to use these elections to send a strong message of discontent to the government.
"I feel like France's public sector is being sabotaged," said Elsa Quinette, in Paris.
"What the government is doing is so serious, I just had to speak out."
Others said they wanted the government to stay.
Mr Chirac has spent the past week weighing up his options.
He will have to choose whether to keep Mr Raffarin in his post and use him to push through the next round of reforms - this time to public healthcare - or whether to appoint a new prime minister.
The problem with that is that the most obvious candidate, France's popular Interior Minister Nicholas Sarkozy, is known to want Mr Chirac's job, correspondents say.
So the president may prefer to keep on his unpopular but loyal prime minister to plough ahead with the next round of difficult reforms, despite voters' discontent.