INCOMING president François Hollande said France had "chosen change" in electing him ahead of Nicolas Sarkozy and promised to be the "president for everyone" saying "the change starts now".
Speaking in his home town of Tulle in the Corrèze, he told the cheering crowd he wanted to "give back hope" and said there had been "too many wounds, too many breaks, too many splits, too many cuts".
He then flew to Paris to address tens of thousands of supporters who had flocked to the Place de la Bastille in central Paris to mark the first Socialist president in 31 years, the last one being François Mitterrand.
But he did not stay long after addressing the jubilant crowd, saying he had to work at what was a "difficult time for the country and for Europe".
The party, however, continued into the night. It also went on in major towns and cities across the country with celebrations at the Capitole in Toulouse, the Place de la Victoire in Bordeaux, the Transbordeur hall in Lyon and the Grand-Place in Lille.
Hollande said the duty of a president of the republic was to pull everyone together: "This evening there are no two sides of France."
His two main challenges would be delivering justice for all and helping young people into work. He said his aim would be for growth.
He added, "In many European countries this victory will be received with relief and hope; as a sign that the austerity-only approach to the crisis is not inevitable."
He aims to rebuild France's industry, reduce its public spending, defend its welfare system and give priority to education.
He must now build a new government - and decide his new prime minister, with commentators saying Nantes mayor and MP Jean-Marc Ayrault is front-runner because of his links with Germany.
Hollande will officially take over on May 15 as there is a G8 summit in the US on May 21-22. He has already been invited to the White House by President Obama.
He has accepted President Sarkozy's invitation to join him at tomorrow's events to mark the end of the war in Europe.
His win came as voters in Greece also rejected the government that had signed up to Eurozone austerity measures. The euro took an immediate hit against both sterling and the dollar.
Credit ratings agency Standard and Poor's said in a statement that the election had "no immediate impact" on its outlook on France - it has lost its AAA rating - and added: "We still believe there is at least a one in three chance that we may lower the long-term rating on France this year or in 2013."
The Socialist victory margin of 51.62% of the vote was narrower than most polls had predicted but, again, was widely publicised ahead of the 20.00 end of polling when foreign websites broke the French legal embargo on giving details. Turnout was 81.03%.
Sarkozy's spokeswoman Nathalie Kosciusko-Morizet immediately said two million blank votes had cost him the election, pointing the finger at Front National leader Marine Le Pen who had said she would lodge a blank vote rather than choosing one of the candidates.
He is the first president in three decades not to gain a second term.