Francois Hollande was hailed tonight as a force for further European integration - despite fears that his differences over economic policy will deepen rifts in Brussels.
The new French president is determined to re-open the EU's new "fiscal treaty" to balance its austerity drive with together commitments on restoring growth.
The move, signalled early on in his presidential election campaign, puts him at odds with German Chancellor Angela Merkel and her insistence on austerity measures as a priority to prop up the eurozone.
But Hollande's arrival in the family on EU leaders is also likely to put him at odds with Prime Minister David Cameron over a tax on financial services - which the UK refuses to consider - and the prospect of a new French assault on the multi-billion pounds a year rebate on Britain's EU budget payments.
EU officials in Brussels believe Hollande will swiftly deploy renewed French clout in the Union to shake up the current carefully-laid economic strategy of tough budget discipline on eurozone economies and enforced austerity on counties needing bail-outs.
Already, before Sunday's French election result, the EU's Economic Affairs Commissioner had signalled a new emphasis on growth, after months in which EU policy had been driven by the Franco-German austerity-driven strategy.
But the so-called "Merkozy" double act of Merkel and Sarkozy is now over and relations are likely to be strained between Berlin and Paris until it becomes clear how far Hollande will go in pushing his French ideas on what the EU's growth pact should look like.
Meanwhile, European Commission President Jose Manuel Barroso issued a formal statement congratulating the new occupant of the Elysee Palace, declaring:
"I know I can count on the personal engagement of Francois Hollande to advance European integration"
Barroso insisted that he shared Hollande's conviction that investment in growth was the way forward and that the EU aim now would be to generate durable economic growth and jobs.
But UK Independence leader Nigel Farage predicted the new Hollande economic approach would split the Franco-German alliance:
"Hollande's election is great news" he said. "It will help drive a wedge between Germany and France, the two of the engines of EU integration.
"The friction between German austerity and French profligacy will increase and bring the eventual demise of the EU even closer."
The Party of European Socialists said the arrival of the new French president was "a victory for progress in France and in Europe."
PES President Sergei Stanishev said: "The election of a socialist to the Elysee is a great day, not only for France, but for Europe.
It is a profound rejection of the Merkozy madness that has choked European economic growth for the last two years".
Martin Schulz, the German president of the European Parliament, commented: "Congratulations to François Hollande. Now it's time for a change of direction in Europe.
"On top of fiscal consolidation, which is necessary, the EU is now ready for a growth policy, which is what the European Parliament has been calling for a long time".
Liberal leader in the European Parliament Guy Verhofstadt said former President Sarkozy's stand on the eurozone crisis had been "disappointing and alarming", but Francois Hollande was now promising to "bring his country back into a European dynamic".
Verhofstadt went on: "With him, new prospects will open up for the economic governance of the eurozone".