By Nicholas Vinocur and Lionel Laurent
PARIS (Reuters) - Giddy revellers thronged Paris's Bastille square on Sunday as Socialist Francois Hollande swept to a presidential election victory, erupting into cheers and rejoicing not seen since the party's only other elected president won power 31 years ago.
Tens of thousands of Hollande supporters gave a huge cry of joy as a giant screen showed him as victor, echoing the spontaneous street party in the same historic venue that followed Francois Mitterrand's 1981 election.
The crowd, many too young to remember Mitterrand's triumph, spilled onto the steps of the Bastille Opera House - a modernist legacy of the Mitterrand era - clutching flags emblazoned with the Socialist logo of a clenched fist around a rose.
Some activists brandishing Communist Party flags and pink and purple balloons clambered up the base of a column erected on the site of the Bastille prison stormed during the 1789 French revolution. The monument commemorates another revolt, in 1830, which finally toppled the restored Bourbon monarchy.
"I was six when Mitterrand was elected. I remember my parents being so happy. Now at last we have the beginning of a real social movement," said Fabienne Chabert, 36, a schoolteacher from an eastern Paris suburb.
Revellers blew whistles and some pounded African drums. Reggae music and the rhythms of early 1980s hits from French rock band Telephone regaled the Mitterrand generation.
"I've been waiting for this for 10 years," said laborer Aziz, 31, whose T-shirt featured the words: "No Sarkozy Day".
In a flashback to the days after Mitterrand's victory, when older conservatives feared France would turn Communist and lined up to withdraw their savings from banks, he added: "I only hope the markets don't attack us tomorrow."
The scene recalled the May 10, 1981 election of Mitterrand, Hollande's political mentor, when ecstatic left-wingers stormed the same square, jumping on the roofs of parked cars and partying deep into the night despite thunder and driving rain.
Mitterrand, the only other Socialist to be directly elected president, was a standard-bearer of the European left at a time when the liberal economic policies of U.S. President Ronald Reagan and British Prime Minister Margaret Thatcher were taking hold elsewhere in the West.
Hard leftist Jean-Luc Melenchon tapped into nostalgia for that era of class struggle in March, drawing tens of thousands to the Bastille for a campaign rally at which he denounced capitalism and demanded an end to Europe's austerity policies.
For the centre-left Hollande, victory will be tempered with a sense of realism about tough political choices to come. He inherits a stagnant economy, jobless claims at a 12-year high and anxiety resurfacing about Europe's debt crisis.
"I don't know if Hollande will do any better on the economy than Sarkozy, but I want a president who knows the value of justice and sharing," said Maxime Vissac, 27, a teacher in training in Paris. "I have no illusions about the economy - it's going to be tough all over Europe."
Supporters at Socialist party headquarters in Paris sang a rhythmic chant of "We've won! We've won!" as they watched Sarkozy concede defeat on live television and waited for Hollande, still in his rural political base in Correze, central France.
"We've endured five years of Sarkozy, of excessive (economic) liberalism, and now it's good to know that we are going to have a more democratic society," said Lesley Cassin, 19, a student of economics in Paris.
The atmosphere was equally electric in Hollande's constituency of Tulle, where he spent the weekend.
Drivers pounded their car horns as they drove around the town of 15,000 people. Bystanders erupted in cheers of "Hollande president! Hollande president!"
Even before polling stations closed, the candidate was mobbed by well-wishers who kissed his cheek and shook his hand as he emerged from his car.
For supporters of Sarkozy, the first president since Valery Giscard d'Estaing in 1981 to fail in a re-election bid, Sunday's election represented the end of months of hoping for an upset in the face of unfavorable polls.
His UMP party cancelled plans for a victory celebration in the central Place de la Concorde, where King Louis XVI was guillotined during the revolution.
At a meeting hall in central Paris where the conservative party faithful had gathered, the atmosphere was subdued. Flag-waving stopped and the smartly dressed crowd booed when the results were projected on a giant screen.
"We're disappointed. We're sad. Sarkozy made some very important reforms during five years," said Pierre Aujay, 26.
Others expressed regret that the outspoken and boisterous Sarkozy would be replaced by a man they said they considered uncharismatic and dull. "He has good sides and bad sides but at least he had passion," said Anna Rosetti, 62, a decorator.
"With Hollande, it will all be tepid."
Sarkozy conceded defeat in a speech to them within 20 minutes of polls closing and his supporters rapidly melted away. As they dispersed, one man held up a French tricolor flag with the words "Thank you, Nicolas" printed on it.
Around him a chant sprung up of "Now we are screwed."
(Additional reporting by Alexandria Sage and Marion Douet, writing By Nicholas Vinocur; Editing by Paul Taylor and Alastair Macdonald)