Francois Hollande has vowed to "open a new path" to address Europe's debt crisis after being sworn in as French president amid ceremonies in Paris.
Hollande, who became the country's first Socialist head of state in 17 years on Tuesday, was later forced to turn back when his jet was hit by lightning while on its way to Berlin.
He took off again in a different aircraft, and met with German Chancellor, Angela Merkel.
Hollande replaced the conservative president, Nicolas Sarkozy. Sarkozy handed over the levers of power including the country's nuclear codes and other secret dossiers, before bidding his successor goodbye on the steps of the palace and leaving with his wife Carla Bruni-Sarkozy.
Nicolas Sarkozy earlier greeted his successor to the Elysee Palace.
"I am addressing a message of confidence to the French people. We are a great country that has always risen to its challenges," Hollande said, adding that he would run the country with "dignity and simplicity".
Reporting from Paris, Al Jazeera's Jacky Rowland said that these two words represented a break with the "flashy" style of Sarkozy's presidency, and that Hollande's down-to-earth style "will be more in harmony with ordinary French people."
New prime minister named
Shortly after assuming the presidency on Tuesday, Holland named the head of the Socialist parliamentary bloc, Jean-Marc Ayrault, as his prime minster.
Hollande comes to power with the single currency eurozone bloc teetering back into crisis with fears about Greece's future in the single currency .
Hollande has pledged a growth-led approach to tackling the crisis, setting him at odds with Merkel and his predecessor who had pressed debt-stricken eurozone members to adopt tough austerity measures.
He faces an uncertain reception from Merkel, after saying he wanted to re-open talks on a fiscal pact agreed in March that aims to control European debt by enshrining greater budget discipline.
Merkel has repeatedly insisted since Hollande's election that the pact, signed by 25 of the 27 EU countries and already ratified in some, is not open to renegotiation.
But observers say there could be room for manoeuvre, with Hollande likely to agree to additional stimulus measures without a complete rewrite of the agreement.
In his inaugural speech, Hollande restated his campaign promise to seek to amend a European pact to add growth-boosting measures to deficit-cutting policies that critics say are dampening the bloc's growth prospects.
"To overcome the current crisis, Europe needs a project, needs solidarity, needs growth. I will propose a new pact to our partners which will ally the reduction of public debt, which is necessary, with an economic stimulus
which is essential," he said.
The possible appointment as prime minister of Ayrault, a German speaker with strong contacts in Berlin, could also point to rapprochement between the leaders of Europe's two largest economies.
And with political paralysis in Greece raising the spectre of the country being forced from the eurozone, both Merkel and Hollande will be keen to reassure worried markets they can work together.
Hollande takes over at a time when French debt has swelled to 90 per cent of GDP, the trade deficit is at a record $90bn and nearly one in four young French workers is unemployed. The public sector accounts for 56 per cent of annual output, higher than in any other European country.
Anxious not to lose the down-to-earth image that appealed to voters, Hollande asked for his inauguration ceremony to be kept as low-key as possible.
In a break with tradition, he invited just three dozen or so personal guests to join about 350 officials at the event. Neither his nor his partner Valerie Trierweiler's children attended. Hollande has announced that his official car will be a hybrid Citroen.
He was presented with the official chain of office, a gold collar engraved with the names of all Fifth Republic presidents before being taken on a traditional procession down the Champs Elysees avenue in an open-topped car - despite heavy rain.