Police opened inquiries into several media outlets after national news agency Agence France-Presse and others posted partial scores before polling stations had all closed at 8 p.m.(1800 GMT), the public prosecutor's office said.
Seeking to enforce a 1977 law that imposed a blackout on disclosing results, projections or exit polls before all voting was done, the authorities had threatened fines of 75,000 euros ($99,000) and legal pursuit of any breaches.
The warnings spurred derision and defiance with a profusion of dummy results and fun-poking messages on a micro-blogging network where national frontiers no longer exist.
"Netherlands-Hungary qualify for return leg," said one tweet in a play on the name of Socialist challenger Francois Hollande, top scorer in Sunday's vote, and the origin of second-placed President Nicolas Sarkozy's father.
The march of communications technology has made the law look increasingly like the Maginot Line of anti-tank defenses which France, bloodied by World War One, built on its border with Germany. It failed to prevent Hitler's troops simply driving around the barrier and invading - through Belgium - in 1940.
Some tweets even referred to the coded messages broadcast by General Charles de Gaulle's Free French over Radio London to Resistance fighters in France during World War Two.
Only two of the 10 candidates in Sunday's first ballot go through to a runoff on May 6, in which the conservative Sarkozy will meet Hollande, who remains favorite in opinion polls.
Twitter users had a field day concocting new names for candidates, imaginary news headlines of outcomes and officially unverifiable reports of partial results from remote overseas territories where voting took place on Saturday.
"According to observers returning from Syria, Russian tanks left at dawn, due to arrive in Paris at 20h (8 p.m.)," read one entry, alluding to a possible left-wing victory and closing time at polling stations.
Other aliases for Hollande included "Gouda", "The Flan", a caramel pudding that resembles one of his nicknames, and more transparently, "The Rose of Correze", combining the Socialist party logo with Hollande's rural constituency in central France.
For Sarkozy, they included "platform heels", a reference to Sarkozy's penchant for shoes that give the diminutive president a few extra centimeters in photographs, "Rolex" in a nod to his taste for flashy wristwear, and "Goulash", a Hungarian recipe.
"Daddy's girl" clearly alluded to far-right candidate Marine Le Pen, who took over from her father Jean-Marie last year as head of the anti-immigration National Front.
Firebrand leftist Jean-Luc Melenchon was branded "hot red pepper" by one micro-message sender.
Some messages relayed unofficial partial results or send-ups of result headlines, using candidates' real names, but with their scores blotted out or drowned in a jumble of numbers and characters.
Several publications, some of them relayed by Twitter users, were notified to police by the body in charge of ensuring that voting laws are respected.
They included Agence France-Presse, two Belgian news organizations and a Swiss media outlet, a Belgian journalist and an Internet site that appeared to be based in New Zealand, a spokeswoman at the public prosecutor's office said.
Polling institutes traditionally prepare reliable estimates for their clients, television and radio stations, in the two hours between polling stations closing in most areas at 6 p.m. (1600 GMT) and the late closers in the big cities, opening up a gap when information can leak.
Among the myriad messages with a wartime ring were ones that mocked Sarkozy for his 2007 post-victory cruise aboard the private yacht of multi-millionaire businessman Vincent Bollore: Ran one: "Pink wave turns to tsunami, Bollore yacht in difficulty."