The farmers blasted the stand of the meat brand Charal with fire extinguishers before dumping about 30 kilogrammes of flour on it.
Charal is owned by the agribusiness giant Bigard, which has a turnover of 4.3 billion euros and a payroll of 14,000. Last year during negotiations with farmers the group refused to fix a minimum buying price.
"For every hundred euros it gets (for its meat), Bigard pays only eight to the producer," one of the farmers said, while handing out fake 100 euro notes. "Our job has a cost."
"When you pay between 17 and 20 euros per kilo for meat, you should know that we are only getting two and a half to three euros," said Pierre Vaugarny, secretary general of the National Bovine Federation, using a handheld loudspeaker.
In a statement Charal said it "understands the farmers' difficulies, which are related to structural and short-term problems that can only be solved in a collective manner."
They wanted to "say loud and clear at the stand... that this country's agricultural producers don't feel like citizens," FNSEA secretary-general Dominique Barreau told reporters. "That's the exasperation, that's where we are!"
City dwellers looking to get in touch with their rustic roots flocked to the flagship farm expo against the backdrop of a deeply distressed agricultural sector, which saw farmer frustrations boil over.
Earlier, livestock farmers booed and whistled as Hollande and Agriculture Minister Stephane Le Foll arrived to inaugurate the nine-day event in southern Paris set to attract some 700,000 visitors to the vast Porte de Versailles exhibition centre.
"I hear the cries of distress," said Hollande, who plans to seek re-election in 15 months despite dismal approval ratings. "If I am here today it's to show that there is national solidarity."
France has seen months of nationwide protests, with farmers blocking roads with their tractors and dumping manure outside government offices, generally enjoying broad public support.
At the exhibition centre, excited children gawked at massive cows, giggled at suckling pigs and timidly reached into cages to pet rabbits amid the hay-strewn aisles.
"It's good for little Parisians to see real cows," said homemaker Brigitte Bruneau, 59.
Angelique Mellion, with six-year-old Tao in tow, said she came "to teach my son about agriculture and to taste regional products. So we're both happy."
Despite the widespread despair in the farming sector, exhibitors were loath to boycott the event.
The Salon de l'Agriculture is a must on the calendar of any ambitious politician, and ahead of next year's election, the glad-handing -- and the "stroking of cows' behinds" made virtually compulsory by earthy former president Jacques Chirac -- is the order of the day.
But the FNSEA warned: "It's out of the question for the fair to become a political beauty contest once again."