French farmers protesting over falling food prices threatened to step up blockades of cities, roads and tourist sites on Wednesday, as the government prepared to unveil emergency aid for them.

Farmers have dumped manure in cities, blocked access roads and motorways and hindered tourists from reaching Mont St-Michel in northern France, one of France's most visited sites.

Overnight Tuesday, farmers briefly blocked the A1 motorway, a key artery between Paris and northern France, with around 500 tractors.

The head of the powerful FNSEA farmers' union, Xavier Beulin, said he expected the protests to continue, adding they "could be extended to other regions on Wednesday."

Jean-Pierre Fleury, head of a union representing beef producers, said: "The farmers will not let up, for the simple reason they are fighting for their lives."

A combination of changing dietary habits -- French consumers are eating less meat -- and foreign competition has driven down pork, beef and milk prices.

Farmers blame supermarkets, distributors and the food processing industry for keeping prices low.

Retailers and food industry chiefs promised to raise prices on meat and dairy after a meeting with farmers last month, but the farmers say price hikes in supermarkets have yet to filter down to them.

- 'Exasperation and anger' -

Stung into action, French President Francois Hollande has promised to unveil an emergency plan later Wednesday.

Agriculture Minister Stephane Le Foll has already hinted measures could include tax breaks and debt relief for heavily indebted farms.

The government will also publish an eagerly awaited report by an independent mediator which will decide who is at fault for keeping prices low.

Le Foll has said around 10 percent of farms in France (approximately 22,000 operations) are on the brink of bankruptcy with a combined debt of 1.0 billion euros ($1.1 billion), according to the FNSEA.

FNSEA chief Beulin warned that the farmers were expecting "a bit more than trivial measures" from the government on Wednesday.

He called for "a medium- and long-term plan with ambitions and objectives."

While the farmers' actions have grown more violent over the past few days, anger has been increasing since the beginning of the month, with around 12,000 farmers in total protesting across the country.

"There is a sort of exasperation and anger that has been rising for weeks. You can't say you weren't warned," stressed Beulin.

Although agriculture only accounts for two percent of French gross domestic product, farming provides nearly one million jobs directly or indirectly and politicians pay close heed to the industry's mood.

The dairy industry has suffered a triple shock from the Russian embargo on Western products over the Ukraine crisis, a dip in Chinese demand and the end in April of European milk quotas.

According to recent figures from the French agriculture ministry, the price of milk bought from the producer plunged by 12 percent in May compared with the same month the previous year -- to 303 euros per 1,000 litres.

Meat producers have also been hit by the Russian embargo and are battling strong competition from neighbouring countries such as Spain, Germany and the Netherlands.

Prime Minister Manuel Valls has stressed that "the government cannot act alone". He called on "everyone to live up to their responsibilities", including retailers and the food processing industry.

Hollande has also insisted that consumers play their role, calling on them to "eat French-produced food as much as possible."