* Socialist, conservative activists shocked at elimination
* Macron camp cheers at packed conference hall
* Melenchon supporters bitter, unwilling to back Macron
By Mathieu Rosemain, Dominique Vidalon and Sarah White
PARIS, April 23 (Reuters) - Flag-waving supporters cheered French centrist Emmanuel Macron and far-right leader Marine Le Pen's accession to the second round of France's presidential elections on Sunday night as downcast supporters of France's two main traditional parties quietly slipped out of their near-empty headquarters.
In a packed Porte de Versailles conference hall on the edge of Paris, thousands of well-heeled young fans of the upstart centrist Macron - many of them students voting for the first time - clapped, chanted and hugged one another to loud hiphop and disco music.
Across town, a few dozen supporters of Benoit Hamon, candidate of the defeated outgoing Socialist Party, left a cavernous conference centre in central Paris in total silence.
French and European flags lay on the floor as waiters closed a small bar where only soft drinks had been on offer.
The team running the campaign for Hamon, who had been expected to lose and scored his party's lowest score in its modern history, had not bothered to order champagne.
As the first results appeared on huge television screens at 8 PM (1800 GMT), some of the supporters of conservative challenger Francois Fillon screamed and swore as they realised the fate of their champion.
A young woman member of Fillon's election team burst into tears and left the large room that was by then mostly made up of journalists. The mood had grown heavier among supporters as the time of the results approached.
"If I were confident I wouldn't be drinking Coke and I am drinking Coke here," Senator Roger Karoutchi of Fillon's The Republicans party told reporters just before it became clear that Fillon was out of the running.
Some young conservatives, still in their "Equipe Fillon" T-shirts had sneaked into the Macron rally nearby to ask why people there had preferred Macron over their candidate.
"I came here more out of curiosity than out of a feeling of disappointment. Fillon has called on us to vote for Macron in the second round, so we wanted to hear his speech," said 21-year old biotech student Thomas Joucla.
Le Pen's victory party took place at the Francois Mitterrand sports centre in her flagship town of Henin-Beaumont, northern France. She was the only one of the main candidates to spend the evening outside the capital.
Roars of "Marine Presidente" went up there as the early projected results came through.
Supporters sang La Marseillaise over and over again. One woman repeatedly made the sign of the cross, shouting "I can't believe it, I can't believe it, finally!"
"Tonight is party time," said hairdresser Aurore Cappelle, as some 300 far-right supporters formed conga lines and local mayor Steeve Briois danced to "I love rock and roll".
Backers of the far-left showman Jean-Luc Melenchon were also disappointed that not he, but Le Pen, will challenge Macron now for the presidency.
In a defiant and bitter atmosphere outside a sports bar in northern Paris, where Melenchon held his electoral night, dozens of his fans shouted chants of resistance.
Like Melenchon himself, many of them refused to say whether they would vote for Macron in order to keep Le Pen from winning - a pledge made by both Fillon and Hamon.
Thirty-seven year-old Melenchon voter Fahrid, unemployed, said he would not go and vote in the second round on May 7.
"I am staying home. The game is over, Macron is president already. I have no job. Macron, Le Pen, all the same," he said. (Reporting by Mathieu Rosemain, Matthias Blamont, Sybille de la Hamaide, Sarah White and Dominique Vidalon; Writing by Geert De Clercq; Editing by Andrew Callus)