Hollande's first day in power was marred by a downpour as he rode by open-top car through Paris to mark his investiture, and lightning that hit his presidential jet, forcing him to make a U-turn en route to Germany and switch to a smaller Falcon jet.
After a handover with predecessor Nicolas Sarkozy, Hollande began his five-year term with a bang, naming a new German-savvy prime minister in addition to swearing-in ceremonies before rushing off to meet German Chancellor Angela Merkel.
In his inaugural speech to some 400 guests at the Elysee presidential Palace, the 57-year-old Hollande said he would try to temper the German-inspired austerity focus of a European pact on deficit-reduction by securing commitments to measures that could boost economic growth.
"I'll say what I think. The situation is serious," he said. "We have to find solutions together so that austerity does not become our only fate, so that growth returns and so that we can find solidarity over the concerns we have over Greece."
In a veiled swipe at Sarkozy, who some faulted for being too controlling and too impulsive, Hollande said he would run a "dignified", "simple" and "sober" presidency and ensure parliament played its full role.
"I will set the priorities but I will not decide for everyone, on everything and (be) everywhere," Hollande said.
Sarkozy and his wife, singer and former model Carla Bruni, left the Elysee Palace under a bright sky but rain poured down when the new president left for the traditional inauguration drive in an open-topped car.
Soaked to the skin, Hollande grinned at bystanders as his car crawled up the Champs Elysees avenue to the Arc du Triomphe, where he relit the flame at the memorial of the Unknown Soldier and laid a wreath.
He changed suits and ate lunch before making a number of other sorties to mark the day, paying tribute to a politician who introduced free, secular education in the 19th century and to physicist Marie Curie who twice won a Nobel prize.
His meeting with Merkel was delayed after lightning forced him to go back to Paris and board a different jet for Berlin, according to French officials.
The rest of the government should be unveiled on Wednesday, before a first cabinet meeting on Thursday, and before Hollande flies to Washington to meet President Barack Obama and attend G8 and NATO summits at Camp David and Chicago.
Hollande, whose election comes as the euro zone is teetering back into crisis over fears about Greece's future in the single currency, will give his first presidential news conference in Berlin in the evening, flanked by Merkel.
His comments will be keenly watched by financial markets eager for reassurance that his push to tack pro-growth policies onto Europe's budget discipline treaty will not sour the start of his relationship with Merkel.
"To overcome the crisis Europe needs projects, solidarity and growth," he said in his inaugural speech. "I will propose to our partners a new pact to combine the necessary reduction in public debt with essential economic stimulus."
He also said he would press the need for Europe to protect its interests, especially regarding reciprocity in trade.
Any indications on initial economic policy will be scrutinised both outside France and inside, where frustration over rampant unemployment and a sickly economy were key factors behind conservative Nicolas Sarkozy's defeat.
For prime minister, he named former German teacher Jean-Marc Ayrault, a 62-year-old who has held elected public office since the mid-1970s, been a member of parliament since 1986 and met advisers to Germany's Merkel last year in an endeavour to build ties with her conservative government.
Hollande named as chief of staff Pierre-Rene Lemas, whom he has known since they attended the ENA civil service academy at the end of the 1970s. Lemas's deputy will be Emmanuel Macron, an investment banker in his 30s who will be an economic adviser.
Hollande was officially sworn in a president just before 11 a.m. (0900 GMT) after Sarkozy greeted him on the steps of the Elysee and took him inside to hand over the country's nuclear codes and other secret dossiers.
Anxious not to lose the "Mr Normal" image that appealed to voters tired of his showman predecessor, Hollande had asked for the inauguration ceremony to be kept as low-key as possible.
He invited just three dozen or so personal guests to join some 350 officials at the event and neither his nor his partner Valerie Trierweiler's children attended the ceremony.
Still, the man who until recently went to work on a scooter was presented with the official chain of office, a gold collar weighing nearly a kilogramme and engraved with his name and the names of the six previous presidents of the Fifth Republic.
After his rainsoaked victory lap, a sodden Hollande walked back up the red-carpeted Elysee steps to eat his first lunch as president with a group of former Socialist prime ministers.
When a Reuters reporter in the courtyard called out: "You are not afraid of the rain Mr. Hollande?", he laughed and replied: "I am not afraid of anything."
